The CT600 now needs software, wherever the company’s owner lives

The return that used to be free to file

The CT600 is the form number of the Company Tax Return, the annual corporation tax return that a UK company sends to HM Revenue and Customs. For more than a decade a small company with simple affairs could prepare its accounts and its CT600 together on a free government website and file both at once. Many owners who live abroad and run a modest UK company did exactly that, without an accountant.

That route closed on 31 March 2026. HMRC's announcement that the online accounts and Company Tax Return service is closing set out the consequence: accounts can still go to Companies House by its own web service, by paper or through software, but the Company Tax Return to HMRC must go through software. Five days later, the penalties for filing it late doubled.

The two changes landed on the owners least equipped to absorb them. An owner in Dubai, Lagos or Lisbon with a UK company that trades lightly, or barely at all, now needs software that produces tagged accounts, a tax computation and a return, or someone who operates it for them. The obligation itself has not changed. What has changed is that doing it alone, for free, is no longer an option, and the cost of missing it has risen.

What the CT600 is and who has to file it

The return is the company's own self-assessment of its corporation tax, under Schedule 18 to the Finance Act 1998. It sets out the taxable profit or loss for one accounting period, the tax due on it and the claims and reliefs the company makes.

The trigger is a notice to deliver. HMRC's guidance on Company Tax Returns is direct: a company must file if it receives a notice to deliver a Company Tax Return, and must still file if it makes a loss or has no corporation tax to pay. The return is not a declaration of profit. It is a statement of the company's position, and a loss is a position.

Registration comes first. A company must register for corporation tax within three months of starting to do business, which includes buying, selling, employing or renting premises, not only earning income, as set out in the note on registering a UK subsidiary for tax before it trades. Registration is what leads HMRC to issue the notice.

Residence of the owner changes nothing. A UK-incorporated company is UK resident under section 14 of the Corporation Tax Act 2009 wherever its directors and shareholders live, so it files a CT600 on its worldwide profits. A board that moves to Dubai does not move the company with it, as set out in the note on company exit tax.

Some non-UK companies file too. A company incorporated abroad files a CT600 where it carries on a trade in the UK through a permanent establishment, and a non-resident company with UK rental income has been within corporation tax, and filing CT600s, since 6 April 2020, as examined in the note on non-resident companies holding UK property.

What goes with the form. A CT600 is not a single page. It travels with the company's statutory accounts and a tax computation reconciling accounting profit to taxable profit. HMRC's XBRL guide for businesses records that, since legislation in force from 1 January 2010, the accounts and computations of an online return must be tagged in iXBRL, a machine-readable format that software produces and a spreadsheet does not.

Deadlines run from the end of the accounting period

The return and the tax have different dates, and the earlier of the two is the payment, fixed for most companies by section 59D of the Taxes Management Act 1970. A company pays its tax before it files the return that calculates it.

The accounting period cannot exceed 12 months. HMRC's page on accounting periods for corporation tax states the rule and its consequence: where the accounts cover more than 12 months, the company files two returns. A new company's first accounts usually run from incorporation to the end of the month of the first anniversary, so they often cover slightly more than a year, and the first accounting period may also start later, when the company comes within the charge to tax. The result is two CT600s from one set of accounts, each with its own deadlines, and a common first-year error is to file only one.

Lengthening the year needs notice. A company that extends its financial year must update its accounting period dates with HMRC before the original filing date of the return, or risk a late filing penalty on a return it did not know it owed.

Corrections have a window. A filed return can usually be amended within 12 months of the filing deadline, through software or in writing, according to HMRC's guidance on making changes. After that, overpaid tax can sometimes be recovered by an overpayment relief claim, and underpaid tax should be disclosed.

How the CT600 is filed since April 2026

The filing route now has one door. HMRC publishes a list of commercial software suppliers for corporation tax whose products it accepts. Some suppliers also produce the iXBRL accounts and computations. A paper return is allowed only where the company has a reasonable excuse for not filing online, or is filing in Welsh.

In practice there are three ways to meet the obligation.

1. An agent files for the company. An accountant or tax agent prepares the accounts, the computation and the CT600 in their own software, files the return with HMRC and the accounts with Companies House, and gives the company the dates. This is the usual route for an owner abroad.

2. The company buys software and files itself. Filing products exist at different prices, including some aimed at small and dormant companies. The owner then has to produce tagged accounts that match the figures in the return, which is where self-filers most often fail.

3. Accounts to Companies House separately, CT600 by software. For now, accounts can still go to Companies House by its web service or on paper, but the return to HMRC cannot. Companies House will also require software filing of accounts from 1 April 2028 under the Economic Crime and Corporate Transparency Act 2023, as set out in the note on the two filing calendars of a UK company.

The search behind "how to file a CT600 without software" has a short answer. Since 1 April 2026, a company cannot, unless it qualifies to file on paper.

The penalties doubled from 1 April 2026

HMRC's guidance on penalties for late filing now shows the higher amounts, and the government's policy paper on increases to corporation tax late filing penalties confirms that they apply to returns with a filing date on or after 1 April 2026.

Two features make these penalties harder than they look.

The determination cannot be appealed. Once a return is six months late, HMRC issues a tax determination, its own estimate of the corporation tax due. There is no appeal against it. The only way to displace it is to file the return.

A dormant company is not exempt. The flat-rate penalties apply whether or not tax is due, so a company with no profit that ignores a notice to deliver pays £200 and then another £200 for a return that would have shown nothing. A reasonable excuse appeal is possible, but only after the return is filed.

Interest runs separately on tax paid late, from the payment date of 9 months and 1 day. Companies House penalties for late accounts run alongside, on their own scale, and the two regimes do not coordinate.

What the rate depends on

The tax on the return depends on the profit and on how many companies the company counts as associated, under the small profits rate and marginal relief rules in Part 3A of the Corporation Tax Act 2010. According to HMRC's page on corporation tax rates:

• 25% is the main rate, for profits above £250,000.
• 19% is the small profits rate, for profits of £50,000 or less.
• Marginal relief reduces the effective rate between £50,000 and £250,000.
• Both limits are reduced proportionately for an accounting period shorter than 12 months, and divided by the number of associated companies plus one.

The associated companies rule is the one that catches corridor owners. Companies under common control count as associated wherever they are resident, so a UK company whose owner also controls an operating company in Dubai and a property company in Jersey may have its £50,000 limit divided by three. The CT600 asks for the number of associated companies, and an owner abroad who answers it from the UK perspective alone gets the rate wrong. The Budget on 28 October 2026 left these rates and limits unchanged.

The supplementary pages that matter abroad

The main CT600 is supported by supplementary pages, each filed only where it applies. HMRC lists them among its corporation tax forms. Four are the ones a UK company with an owner or a business abroad is most likely to need.

A UK company with a foreign subsidiary that the UK controls will meet CT600B, and the substance of the controlled foreign company question is examined in the note on why a simple UAE free zone fails the CFC rules. The page with the largest bill attached, for owners abroad, is CT600A.

The director's loan an owner abroad does not notice

An owner who lives outside the UK often takes money from the UK company informally: a transfer to a personal account when needed, recorded later as drawings. In the company's books that is a loan to a participator, and it has a corporation tax consequence of its own. HMRC sets out the rules on director's loans.

The section 455 charge. Where a close company has lent money to a shareholder and the loan is still outstanding nine months after the end of the accounting period, the company pays corporation tax at 33.75% of the outstanding amount under section 455 of the Corporation Tax Act 2010, or 32.5% for loans made before 6 April 2022. The charge is reported on CT600A and is due with the company's corporation tax. Interest is added until the tax is paid or the loan is repaid.

The charge comes back, slowly. Once the loan is repaid, written off or released, the company can reclaim the tax under section 458 of the same Act, but not the interest. The relief is not due until 9 months and 1 day after the end of the accounting period in which the repayment happened, and the claim must be made within four years. A company that pays the charge in one year can wait almost two years to see it again.

Recycling does not work. Where a loan of more than £5,000 is repaid and a new loan of £5,000 or more is taken within 30 days, or a loan of more than £15,000 is repaid under arrangements to borrow again, the charge applies to the original loan as though it had not been repaid, under the anti-recycling rules in section 464C.

The personal side is separate. A loan above £10,000 at any time in the year is a benefit in kind for a shareholder-director, and a loan that is written off has its own income tax and National Insurance consequences. For an owner who is not UK resident the personal side needs to be worked out against the UK rules on non-residents and the treaty with the country of residence, which is why the choice between salary, dividend and loan belongs at the start of the year, as set out in the note on paying yourself from a UK company after moving to Dubai.

A dormant company still has a return to deal with

A company that is not trading is usually dormant for corporation tax. HMRC's guidance on dormant companies treats a new company that has not started trading, and a company that has stopped trading with no other income, as dormant, and notes that trading includes buying, selling, renting property, advertising, employing someone or receiving interest.

The trap is the order of events. If the company has already filed a CT600, or has received a notice to deliver, it must file a return online showing that it was dormant for the period. Only once HMRC has been told the company is dormant does the obligation to file stop, and it restarts with any further notice. The company must still file annual accounts and a confirmation statement with Companies House throughout. A dormant company with a notice to deliver and no return pays the same £200 as a trading one.

Where the CT600 goes wrong for owners abroad

One return for a first period that needed two. The first accounts ran for 13 months, and only one CT600 was filed. The second return is late from the day the first was filed.

The notice went to an address nobody reads. The registered office is a formation agent's address, the post is not forwarded, and the first the owner hears of the return is the penalty.

Associated companies counted from the UK only. The return claims the full £50,000 small profits limit, and the owner's Dubai and Jersey companies were never counted.

Drawings left as a loan. Money taken through the year is still on the director's loan account nine months after the year end, and the company owes 33.75% of it.

Dormancy assumed, not declared. The company stopped trading, nobody told HMRC, and the notices kept coming.

Software bought, accounts not tagged. The owner bought a return product but filed accounts that did not match the computation, and the return was rejected after the deadline.

What to settle each year

1. Confirm the accounting periods with HMRC. Check that the dates in the business tax account match the accounts, especially in the first year and after any change of year end.

2. Decide who operates the software. An agent or the owner, named before the year end, not after the penalty.

3. Count associated companies across every jurisdiction. Any company under common control counts, wherever it is incorporated.

4. Clear the director's loan account before the nine-month point. Repay it, or convert it to salary or dividend with the personal tax consequences understood, before 9 months after the year end.

5. Pay before filing. Diary the payment date of 9 months and 1 day, three months ahead of the return.

6. Keep the registered office and email live. Notices to deliver and penalties go there.

7. Tell HMRC when the company goes dormant. Then file the dormant return if a notice has already arrived.

Frequently asked questions

What is a CT600?

The CT600 is the Company Tax Return that a UK company files with HMRC for each accounting period, setting out its taxable profit or loss, its corporation tax and any claims or reliefs. It is filed online with the company's accounts and a tax computation in iXBRL format. A company must file one if it receives a notice to deliver a Company Tax Return, even if it made a loss or has no tax to pay.

When is the CT600 deadline?

The return is due 12 months after the end of the accounting period it covers. The corporation tax is due earlier, usually 9 months and 1 day after the end of the period, so the tax is paid before the return is filed. Companies with profits above £1.5 million, divided by the number of associated companies plus one, pay in quarterly instalments instead.

Can I file a CT600 without software?

Not since 1 April 2026, in most cases. HMRC's free online service for accounts and the Company Tax Return closed on 31 March 2026, and the return must now be filed through commercial software that HMRC accepts. A paper return is allowed only where the company has a reasonable excuse for not filing online, or where the return is in Welsh.

What are the penalties for filing a CT600 late?

For returns with a filing date on or after 1 April 2026, the penalty is £200 on the day after the deadline and another £200 at three months. If returns are late three times in a row, each of those becomes £1,000. At six months HMRC estimates the tax and adds 10% of the unpaid amount, and at twelve months a further 10%.

Do I need to file a CT600 for a dormant company?

Only if the company has already filed a return or has received a notice to deliver one. In that case it must file a return online showing it was dormant for the period. Once HMRC has been told the company is dormant, it does not need to file further returns unless it receives a new notice. It must still file annual accounts and a confirmation statement with Companies House.

What is CT600A?

CT600A is the supplementary page for loans and other benefits a close company gives to its participators, usually its shareholder-directors. Where a loan is still outstanding nine months after the end of the accounting period, the company pays corporation tax at 33.75% of it under section 455, or 32.5% for loans made before 6 April 2022. The tax can be reclaimed after the loan is repaid, but not the interest.

Does a UK company owned by a non-resident have to file a CT600?

Yes. A UK-incorporated company is UK resident for corporation tax wherever its owners and directors live, so it registers for corporation tax, files a CT600 for each accounting period after a notice to deliver, and pays tax on its worldwide profits. The owner's residence affects the personal tax on salary, dividends and loans, not the company's own return.

Why does my company have two CT600s for its first year?

Because an accounting period for corporation tax cannot be longer than 12 months. A new company's first accounts often cover slightly more than a year, from incorporation to the end of the month of its first anniversary, so the period is split into two accounting periods, each with its own return and deadlines. The first accounting period can also begin later, when the company starts to do business.

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Critical advisory. The CT600 used to be the filing an owner abroad could do alone, late at night, on a free government form. That ended on 31 March 2026, and the penalties for getting it wrong doubled five days later. The return is not the difficult part. The difficult parts are the ones the form asks without explaining: whether the first year was one period or two, how many associated companies the owner controls abroad, and what the money drawn during the year really was. In my experience the owners who receive penalty notices are rarely negligent.

They are abroad, the post goes to an address nobody reads, and the software they bought assumed a UK accountant. We prepare the accounts, the computation and the CT600 for UK companies owned from abroad, clear the director's loan position before the nine-month point and keep the dates, through our UK accounting service for non-residents. This article is general information about UK corporation tax as at 11 November 2026 and not tax advice; your company's position should be confirmed against its own facts before you act.