CT600 Company Tax Return: what it covers, CT600A and deadlines

Who needs it

• Limited companies and associations that receive a 'notice to deliver a Company Tax Return' from HMRC.
• Companies that made a loss or have no Corporation Tax to pay. HMRC still expects the return.
• Close companies (controlled by five or fewer participators) with a loan to a participator, such as a director-shareholder, that wasn't repaid in the period. They add the CT600A pages.
• Not sole traders or partnerships. They report profits through Self Assessment instead.

What you need before you start

• The company's Corporation Tax UTR and its accounting period dates, which you can check in the business tax account.
• The statutory accounts for the period, approved by the directors.
• A Corporation Tax computation that turns accounts profit into taxable profit.
• The director's loan account balance at the period end, and any repayments made after it, if the CT600A applies.
• Commercial software that can file a CT600, a computation and the accounts with HMRC.

How to file CT600

1. 1Check the accounting period. It can't be longer than 12 months, so accounts covering a longer period need two returns.
2. 2Prepare the statutory accounts and the tax computation. HMRC points out that taxable profit is different from the profit in your accounts.
3. 3Work out which supplementary pages apply. CT600A covers loans to participators; others include CT600C for group relief and CT600L for research and development.
4. 4File online through commercial software. Paper is only for an amendment, a reasonable excuse or filing in Welsh, and goes to Corporation Tax Services, HMRC, BX9 1AX.
5. 5Pay the Corporation Tax by the payment deadline, which is usually 9 months and 1 day after the period ends, well before the return is due.
6. 6File the accounts with Companies House separately, unless your software sends both.

What a Company Tax Return includes

The CT600 is the form itself, but HMRC's guidance is that a full return also includes a Corporation Tax computation and the company's accounts. Software has to be able to send all three. The computation shows how the accounts profit becomes taxable profit, and the CT600 states the tax due at the small profits rate of 19%, the main rate of 25%, or with marginal relief between £50,000 and £250,000 of profit.

Supplementary pages sit alongside it for particular situations. HMRC lists CT600A to CT600P, including group and consortium relief (CT600C), charities (CT600E), research and development (CT600L) and creative industries (CT600P).

CT600A and the s455 charge

A close company is one controlled by five or fewer participators, which describes most owner-managed companies. If it lends money to a participator and the loan is still outstanding at the end of the accounting period, it reports the loan on the CT600A.

Anything not repaid within 9 months of the period end is charged at the s455 rate, which follows the dividend upper rate for the tax year the loan was made: 35.75% for loans made on or after 6 April 2026. Interest runs on that tax until it's paid or the loan is repaid.

The tax comes back once the loan is repaid, written off or released, but not before 9 months and 1 day after the end of the period in which that happened, and you must claim within four years. Within two years of the period the loan was made in, the claim goes on the CT600A; after that, on form L2P. HMRC has extra rules for loans repaid and then quickly borrowed again.

Online, paper and the closed HMRC service

Until 31 March 2026 small companies could file through HMRC's free 'File your accounts and Company Tax Return' service. From 1 April 2026 HMRC expects commercial software, and an agent can file for you.

The paper CT600 is now limited to amending a return already submitted, filing with a reasonable excuse, or filing in Welsh. If online filing failed because of an HMRC IT problem, send form WT1 with the paper return.

Late filing penalties

HMRC charges £200 the day after the filing deadline and another £200 at three months late. At six months it estimates the tax (a 'tax determination', which you can't appeal) and adds 10% of the unpaid tax, and another 10% at twelve months. Late three times in a row, the fixed penalties rise to £1,000 each.

Penalties apply even when there's no tax to pay. You can appeal with a reasonable excuse, but only after the return is filed.

Common mistakes

• Treating the filing deadline as the payment deadline. The tax is usually due three months before the return.
• Leaving out the CT600A when a director-shareholder's loan account was overdrawn at the period end.
• Still looking for HMRC's free filing service. It closed on 31 March 2026; you need commercial software or an agent.
• Filing one return for accounts that cover more than 12 months. HMRC needs two.
• Lengthening the year with Companies House without updating the accounting period dates with HMRC before the original filing date. HMRC warns this can bring a late-filing penalty.
• Making entries in the Northern Ireland boxes. HMRC says not to until a Northern Ireland rate exists.