How to File a CT600 Company Tax Return in 2027 and 2028

The CT600 is the Company Tax Return that UK limited companies file with HMRC. It reports the company's taxable profits for an accounting period and works out how much Corporation Tax is due. Unlike the annual accounts filed at Companies House, the CT600 goes to HMRC, and it has its own deadlines, payment dates and penalties. This guide explains how to file a CT600 step by step: who must file, the separate deadlines for filing and paying, the current tax rates, what goes into the return and how to avoid penalties.

This guide explains how to file a CT600 in 2027 and 2028: who has to file, how accounting periods work, the deadlines for filing and paying, the current tax rates, what goes into the return, iXBRL tagging, supplementary pages, and the penalties for getting it wrong. If your company didn't trade, our guide to dormant company accounts and a nil CT600 covers the simpler route.

This guide is general information, not tax advice. Tax rules and HMRC services change. Check HMRC's Corporation Tax guidance or speak to an accountant for your situation.

Who has to file a CT600

A company must file a CT600 if it is within the charge to Corporation Tax, or if HMRC sends it a notice to deliver a Company Tax Return. In practice that covers:

• every UK limited company that trades or has income, such as interest or rent;
• dormant companies that have received a notice to file from HMRC (many dormant companies can tell HMRC they're dormant instead);
• foreign companies with a UK permanent establishment, such as a branch; and
• some clubs, associations and co-operatives.

Sole traders and partnerships don't file a CT600; they use Self Assessment.

Accounting periods versus financial years

The CT600 covers an accounting period for Corporation Tax, which isn't always the same as the period of the company's accounts.

If your accounts cover more than 12 months, for example the first accounts after incorporation, you file two CT600s: one for the first 12 months and one for the rest. This is one of the most common reasons first-year returns go wrong.

Example: first accounts covering 15 months

A company is incorporated on 1 January 2026 and draws up its first accounts to 31 March 2027, a 15-month period of account. For Corporation Tax there are two accounting periods:

1. 1 January 2026 to 31 December 2026 (12 months)
2. 1 January 2027 to 31 March 2027 (3 months)

Each needs its own CT600, with profits split between them according to HMRC's rules.

Deadlines: filing and paying are different

Corporation Tax has two separate deadlines, and many companies miss the earlier one.

So for an accounting period ending 31 March 2027, the tax is usually due by 1 January 2028, and the return by 31 March 2028. Interest runs on tax paid late, even if the return itself isn't late yet.

Corporation Tax rates

Since 1 April 2023, Corporation Tax depends on the company's profits:

The £50,000 and £250,000 limits are reduced if the accounting period is shorter than 12 months, and divided between associated companies (companies under common control). Marginal relief gives an effective rate between 19% and 25% for profits in the middle band. Check HMRC's current rates page for the rates that apply to your financial year.

What goes into a CT600

A complete Company Tax Return is more than the form itself. You file three things together:

1. The CT600 form: company details, the accounting period, income, deductions, tax due and any reliefs claimed.
2. The company's accounts for the period, in iXBRL format.
3. Tax computations: a working that starts with the accounting profit and adjusts it to taxable profit, also in iXBRL.

From accounting profit to taxable profit

Your accounts show a profit; HMRC taxes an adjusted profit. Common adjustments include:

Getting capital allowances right is often the biggest tax saving available to a small company, so make sure qualifying equipment purchases are claimed.

Supplementary pages

Depending on the company's activities, the CT600 needs extra pages. The most common are:

Director's loans and section 455 tax

If a director (or other participator in a close company) owes the company money at the end of the accounting period, and it isn't repaid within 9 months and 1 day, the company pays an extra temporary tax under section 455 on the outstanding amount, currently 33.75%. It's refundable once the loan is repaid, but it ties up cash. Report it on the CT600A.

iXBRL: tagging the accounts and computations

HMRC requires accounts and computations to be filed in inline XBRL (iXBRL), a format that tags each figure so HMRC's systems can read it. Accounting software and tax filing software produce iXBRL automatically. You don't need to tag figures by hand, but you do need software that can produce compliant files.

How to file

Using commercial software

Most companies file using commercial accounting or tax software that prepares the CT600, produces iXBRL accounts and computations, and submits them to HMRC online. HMRC has announced that its own free Company Accounts and Tax Online filing service is closing, so companies that used it need to move to commercial software or an agent. Check HMRC's guidance on filing your Company Tax Return for the current position.

Through an agent

An accountant or filing agent can prepare and submit the return for you, using their software and their HMRC agent authorisation. Our CT600 filing service covers nil and dormant returns and straightforward small-company returns, filed with matching accounts.

What you need before you start

• the company's Unique Taxpayer Reference (UTR);
• Government Gateway login details for the company, or agent authorisation;
• the final accounts for the period;
• records of fixed asset purchases (for capital allowances);
• details of any director's loan account balance at the period end; and
• details of associated companies, for the rate thresholds.

Step-by-step: filing a CT600

1. Confirm the accounting period, splitting a long period of account into two returns if needed.
2. Finalise the accounts for the period.
3. Prepare the tax computation: adjust the accounting profit to taxable profit and work out capital allowances and losses.
4. Complete the CT600, including any supplementary pages.
5. Generate iXBRL accounts and computations through your software.
6. Review the return: company details, period dates, tax due and reliefs.
7. Submit online and keep the HMRC acknowledgement.
8. Pay the tax by the payment deadline, quoting the correct payment reference for that accounting period.

Paying Corporation Tax

• Pay using the Corporation Tax payment reference for the specific accounting period. Using the wrong reference is a common cause of "unpaid tax" letters.
• You can pay by bank transfer, Direct Debit or card through HMRC's online services.
• Large companies pay by quarterly instalments.
• If you can't pay in full, contact HMRC before the deadline about a Time to Pay arrangement.

Penalties for late filing

If the return is late three times in a row, the £100 penalties rise to £500 each. Interest is charged on late payment separately. These are HMRC penalties, separate from the Companies House penalties for late accounts. See our guide to avoiding late filing penalties.

CT600 and Companies House accounts: how they fit together

The same set of accounts is used for both, but the deadlines and penalties are completely separate. Our guide on how to file annual accounts explains the Companies House side.

Dormant and nil returns

If the company had no income and no taxable activity, it may not need to file a CT600 at all, unless HMRC has sent a notice to file. If HMRC has sent a notice, file a nil return for the period. Tell HMRC if the company is dormant, so it stops expecting returns. Our dormant accounts and nil CT600 guide explains how.

Common mistakes

1. Paying late because the filing deadline is later. Payment is due 3 months before the filing deadline for most companies.
2. Filing one return for a long first period. Accounting periods can't exceed 12 months.
3. Missing capital allowances, so the company pays more tax than it should.
4. Ignoring an overdrawn director's loan account, leading to unexpected section 455 tax.
5. Using the wrong payment reference, so HMRC shows tax as unpaid.
6. Forgetting associated companies, which reduce the rate thresholds.
7. Treating a dormant company as needing nothing, when HMRC has sent a notice to file.

Practical scenarios

A first-year consultancy

A one-director consultancy incorporated on 1 March 2026 and made up its first accounts to 31 March 2027. It filed two CT600s (1 March 2026 to 28 February 2027, and March 2027), claimed capital allowances on a laptop and office equipment, and paid each period's tax by its own payment deadline.

A director's overdrawn loan

A company's director had drawn £20,000 more than their salary and dividends by the year end. The accountant reported the loan on CT600A. The director repaid it within nine months, so no section 455 tax became payable.

A dormant company with a notice to file

A holding company didn't trade but received a notice to file from HMRC. It filed a nil CT600 with dormant accounts and told HMRC the company was dormant, so no further notices were issued.

Starting a company?

If you're setting up a new UK company, register it for Corporation Tax within three months of starting to trade. Biz E-Formations Inc. handles UK company formation, registration for Corporation Tax and help opening a business bank account, so your first CT600 starts on the right footing.

Checklist

How to file a CT600: worked example with figures

Here is a simplified example of how a small company gets from its accounts to the tax on its CT600. The figures are illustrative only.

Because taxable profit is between £50,000 and £250,000, the company pays the main rate with marginal relief, giving an effective rate between 19% and 25%. Software works the marginal relief out automatically from the profit, the length of the period and the number of associated companies.

Without the capital allowances claim, taxable profit would have been £6,500 higher. That's why tracking fixed asset purchases through the year matters.

Losses: what happens if the company made a loss

If the company makes a trading loss, there's no Corporation Tax to pay for that period, and the loss can be used to reduce tax in other periods:

• Carried back against profits of the previous 12 months, which can produce a tax refund;
• Carried forward against future profits, subject to the loss restriction rules for larger amounts; or
• Group relief, surrendered to another company in the same group.

You still file a CT600 for a loss-making period, and you claim the loss treatment on the return. Filing on time protects the company's ability to use the loss later.

Making a claim for reliefs and allowances

The CT600 is also where the company claims many reliefs. The most common for small companies are:

R&D claims now require additional information to be submitted to HMRC, and in many cases advance notification. Take specialist advice before claiming.

Amending a CT600 after filing

If you spot a mistake after filing, you can usually amend the return within 12 months of the filing deadline. Make the correction in your software and resubmit the full return. If the deadline for amendments has passed and the company has overpaid tax, it may still be possible to make an overpayment relief claim, subject to time limits.

If HMRC opens an enquiry into the return, it will write to the company. Respond promptly and keep the working papers behind the computation, because HMRC can ask to see them.

Records you must keep

Companies must keep the records behind their CT600 for at least 6 years from the end of the accounting period, and longer in some cases. That includes:

• bank statements and payment records;
• sales and purchase invoices;
• receipts for expenses;
• details of assets bought and sold;
• director's loan account records; and
• the tax computations and working papers.

Digital records are fine, as long as they're complete and you can produce them if HMRC asks.

Corporation Tax registration for new companies

A new company must tell HMRC when it becomes active for Corporation Tax, usually when it starts trading, within three months. HMRC generally sends the company's Unique Taxpayer Reference (UTR) soon after incorporation. If you don't register on time, you risk penalties and missed deadlines for the first return.

Questions to ask your accountant

Before your accountant files, ask:

1. Have we claimed every capital allowance available?
2. Is the director's loan account clear at the period end, or will section 455 tax apply?
3. Are there any associated companies that change our rate thresholds?
4. Would carrying back a loss produce a refund?
5. What's the payment deadline, and which payment reference should we use?
6. Has the return been filed with iXBRL accounts and computations that match the Companies House accounts?

Non-resident directors and the CT600

A UK company owned and run from abroad still files a CT600 like any other UK company, because a company incorporated in the UK is generally UK tax resident. Practical points for non-resident directors:

• Make sure HMRC has a correspondence address that reaches you, as notices to file and penalty letters are sent by post.
• Appoint a UK accountant or agent who can file on the company's behalf and deal with HMRC.
• Check whether your home country also taxes the company's profits or your income from it, and whether a double tax treaty applies.
• Keep the company's bank account and bookkeeping in the company's name, so the accounts and computations are straightforward.

If HMRC letters keep going astray, problems escalate quickly: estimated assessments, penalties and, eventually, debt collection.

Key takeaways

• File a CT600 for every accounting period of up to 12 months; a longer first period needs two returns.
• Pay Corporation Tax within 9 months and 1 day; file the return within 12 months.
• Small profits pay 19%, profits over £250,000 pay 25%, with marginal relief in between.
• Claim capital allowances and check the director's loan account before the period end.
• File with iXBRL accounts and computations through commercial software or an agent.

Before you start: a quick readiness check

Before you or your accountant start the return, confirm four things: the company's UTR and online access work, the accounts for the period are final and approved, every fixed asset bought in the period is listed for capital allowances, and the director's loan account balance at the period end is known. With those ready, most small-company CT600s can be prepared and filed in a single sitting.

Related guides

• How to file dormant company accounts and a nil CT600
• How to file annual accounts for a UK company
• How to avoid Companies House late filing penalties
• How to file a confirmation statement

The bottom line

The CT600 is the company's annual tax return to HMRC. File one for every accounting period of up to 12 months, with iXBRL accounts and computations, within 12 months of the period end, and pay the tax within 9 months and 1 day. Claim capital allowances, watch the director's loan account, and use commercial software or an agent now that HMRC's free filing service is closing.