CT600 return filing deadline and late filing penalties

The Company Tax Return – form CT600 with its accounts and computations – must reach HMRC 12 months after the end of the accounting period. It is the last of the annual company deadlines, which makes it tempting to leave. That is a mistake: by the time the return is due, the tax should already have been paid three months earlier.

The filing deadline

The return must be filed online, in iXBRL format, with the statutory accounts and tax computations attached. A company must file even if it has no tax to pay, or has made a loss, once HMRC has issued a notice to deliver a return.

Late filing penalties

HMRC’s penalties for late Company Tax Returns increase the longer the return is outstanding:

• 1 day late: £100
• 3 months late: a further £100
• 6 months late: HMRC estimates the tax and charges 10% of the unpaid tax
• 12 months late: a further 10% of the unpaid tax

If a return is late three times in a row, the £100 penalties rise to £500 each. These are in addition to late payment interest.

Two returns for one set of accounts

A company’s period for Corporation Tax cannot exceed 12 months. When a company prepares accounts for a longer period – very common with first accounts – you need two CT600s: one for the first 12 months and one for the remainder. Both are usually due 12 months after the end of their own period, and both have their own payment date. The second return is easy to overlook if you only diary one deadline per set of accounts.

Dormant companies

A dormant company does not need to file a CT600 unless HMRC has sent a notice to deliver a return. Tell HMRC when a company becomes dormant so that it stops issuing notices. If a notice has been issued, the return is still required.

Practical tips for practices

1. File the CT600 when you finalise the accounts. Once the computations are done there is rarely a reason to wait 12 months, and filing early removes the risk.
2. Keep the accounting period right. In Deadlio, both the CT payment and the CT600 date follow the next period end, so a changed year end updates both.
3. Check HMRC’s records against yours. If HMRC thinks the period ends on a different date, notices and penalties will follow its dates, not yours.
4. Remember LLPs and charities. LLPs do not file CT600s, and charities generally file only where they receive a notice. Deadlio only shows Corporation Tax dates for client types that need them.

Related guides

• Corporation Tax payment deadline
• Annual accounts filing deadline
• Self Assessment deadlines
• See it in practice: The £375 surprise: a new company’s first accounts
• All deadline guides

This guide is general information for accountancy practices and is not a substitute for checking the current rules on GOV.UK. Last reviewed October 2026.