Written by Hammad Rasool MAAT·AAT licensed accountant·Updated November 2026

What the CT603 is

HMRC sends a notice to deliver a Company Tax Return, usually marked CT603, shortly after the end of each accounting period. It names the period the return must cover. Once the notice has been issued, the company must file a return for that period.

GOV.UK is clear that you must still send a return if the company makes a loss or has no Corporation Tax to pay.

The 2 deadlines, and why they catch people out

  • Pay the Corporation Tax: usually 9 months and 1 day after the end of the accounting period.
  • File the Company Tax Return: 12 months after the end of the accounting period.

The payment date comes before the filing date. In practice you need the accounts and the tax calculation finished within 9 months so that you know what to pay.

Example

A company has a year end of 31 March 2026. Its Corporation Tax is due by 1 January 2027. Its Company Tax Return is due by 31 March 2027. Its accounts are due at Companies House by 31 December 2026, which is a separate deadline with separate penalties.

What goes into the return

The return is made up of the CT600 form, the company accounts and the Corporation Tax computation, which shows how the taxable profit was worked out from the accounts. It is filed online, and the accounts and computations must be in the tagged format HMRC requires, so in practice it is done with commercial software or by an accountant.

Penalties for filing late

  • 1 day late: £200.
  • 3 months late: another £200.
  • 6 months late: HMRC estimates the Corporation Tax bill and adds a penalty of 10% of the unpaid tax.
  • 12 months late: another 10% of any unpaid tax.

If the return is late 3 times in a row, the £200 penalties rise to £1,000 each.

Key point: Companies House and HMRC are separate. Filing accounts at Companies House does not file your Company Tax Return, and each body charges its own penalties.

If the company is dormant or has stopped trading

Do not ignore the notice. Tell HMRC that the company is dormant. Until HMRC has confirmed that it does not expect a return, the notice stands and penalties can be charged.

If the period on the notice looks wrong

This often happens in the first year, because a first set of accounts usually covers more than 12 months while a Corporation Tax period cannot be longer than 12 months. The company then has to file 2 returns for its first set of accounts. If the dates on your CT603 do not match your accounts, get advice before filing.

Common questions

Do I have to file if the company made no profit?
Yes. GOV.UK states that you must still send a return if you make a loss or have no Corporation Tax to pay.
When is the Company Tax Return due?
12 months after the end of the accounting period it covers. The tax itself is usually due earlier, at 9 months and 1 day.
Is the CT603 the same as the CT600?
No. The CT603 is the notice HMRC sends asking for a return. The CT600 is the return form the company sends back.
Is the Companies House deadline the same?
No. Accounts for a private company are normally due at Companies House 9 months after the year end. That is separate from the HMRC return.

Checked against GOV.UK on 10 November 2026: Company Tax Returns, Penalties for late filing.

About the author

Hammad Rasool MAAT is an AAT licensed accountant based in Bolton. TaxTune Accountants looks after sole traders, landlords, contractors and limited companies in Bolton, Wigan, Bury, Salford, Oldham, Stockport and Manchester, and online across the UK.