CT603 notice to deliver a Company Tax Return: what to do

In short: A CT603 is HMRC's "notice to deliver a Company Tax Return". It requires the company to file a Company Tax Return (CT600) for the period printed on it, even with a loss or no tax to pay (GOV.UK: Company Tax Returns). The return is normally due 12 months after the period ends, or 3 months after the notice if later. Taxley files the CT600 for £49.95 with micro-entity accounts, then files the accounts at Companies House once the return is paid and finished.

Take the 30-second check to see whether Taxley fits your company, or work out your dates with the free Corporation Tax deadline calculator.

Key facts (checked on 4 October 2026)

What is a CT603 notice to deliver a Company Tax Return?

A CT603 is the form HMRC uses for its formal notice requiring a company to deliver a Company Tax Return. HMRC's manual calls it the "notice to deliver" and says HMRC's Corporation Tax system issues it automatically about one month after the end of each accounting period (HMRC manual COM130130).

The power comes from the Finance Act 1998: an officer of Revenue and Customs may by notice require a company to deliver a return, which must reach that officer by the filing date and include a declaration that it is correct and complete (FA 1998 Sch 18 para 3). The CT603 isn't a tax bill. HMRC's manual says it shows the HMRC officer dealing with the company and the period for which returns are required, and GOV.UK lists it as one place to find the company's 17-character payment reference for that period (GOV.UK: Pay Corporation Tax). It is a different letter from the CT41G, which gives a new company its Unique Taxpayer Reference when it is set up.

Which period does a CT603 cover?

The period printed on it, which is never longer than 12 months (HMRC manual COM130130). The company files a separate return for each accounting period that ends in that period, so one notice can call for two returns, or for a return covering months when the company was dormant (HMRC manual CTM93020).

What a CT603 requires, by what happened in the notice period

Two of HMRC's cases catch directors out. Accounts covering more than 12 months need two returns, because a Corporation Tax accounting period can't be longer than 12 months (GOV.UK: Accounting periods). And a company dormant for the first part of the notice period owes a return for the dormant months before its accounting period began (HMRC manual CTM93020). The same page tells HMRC staff not to insist on a return when satisfied a company was dormant throughout. That is HMRC's decision, not the company's, so don't ignore a notice on the strength of it.

When is a Company Tax Return due after a CT603?

On the filing date, the last day of whichever period ends last: 12 months from the end of the return's period; 12 months from the end of the accounts if they cover 18 months or less, or 30 months from their start if longer; or 3 months from the date the notice was served (FA 1998 Sch 18 para 14).

HMRC counts these periods in a fixed way (HMRC manual CTM93040). Twelve months from the end of a period runs to the same-numbered day a year later, with special rules for 28 and 29 February. The 3 months after a notice start the day after it was served, so a notice served on 15 January makes the date 15 April. In one of HMRC's examples, a year ended on 31 December 2018 and the notice was served on 31 October 2019, so the return was due on 31 January 2020 (HMRC manual CTM93050). Because HMRC normally issues the notice about a month after the period ends, the 12-month rule decides for most companies, and a late notice only ever makes the date later.

What do CT603 filing dates look like in practice?

Three examples, worked out with the paragraph 14 rules and HMRC's counting method, show when the notice date matters. With a notice issued on time, the 12-month rule decides. A notice served late moves the filing date later. Accounts longer than 12 months give the first return the later date of the two.

Calculated filing dates for three example notices (illustrations; the period on each notice is 12 months)

In example A, 3 months from the notice only reaches 5 August 2026, so the 12-month date stands. In example B, 3 months from the 20 November notice runs past 31 December 2026 to 20 February 2027, but the tax doesn't move: it is due the day after 9 months from the period end (TMA 1970 s.59D), before the notice even arrived. Example C lengthened its year to 31 March 2026, so its accounts form two accounting periods and both returns are due 12 months after the accounts end (HMRC manual CTM93050). GOV.UK adds that such a company must update its dates with HMRC before the original filing date, here 31 December 2026 (GOV.UK: Accounting periods).

How do you respond to a CT603 notice?

Check the dates, work out both deadlines, sort out any dormant period or wrong dates with HMRC, pay the tax, then file the return and check that HMRC accepted it. HMRC's own online filing service closed on 31 March 2026, so returns now go through commercial software (GOV.UK: HMRC filing service closure).

Step 1: Compare the notice period with the company's accounts

Set the period printed on the CT603 beside the company's accounts. The company files a separate return for each accounting period ending in the notice period (HMRC manual CTM93020). Keep the notice and note when it was served, because a late notice can move the filing date.

Step 2: Work out the filing date and the payment date

Take the latest of 12 months from the end of each accounting period, 12 months from the end of accounts up to 18 months long, and 3 months from the date the notice was served (FA 1998 Sch 18 para 14). The tax is due 9 months and 1 day after the period ends (GOV.UK: Pay Corporation Tax). Taxley's free deadline calculator gives both dates from the period end, without the notice date.

Step 3: Tell HMRC if the dates are wrong or the company was dormant

If the period on the notice doesn't match the company's accounting periods, tell HMRC through its Corporation Tax enquiries service (GOV.UK: Accounting periods). If the company was dormant, still file a return online: GOV.UK says the return shows HMRC that the company was dormant for that period (GOV.UK: Dormant for Corporation Tax).

Step 4: Pay the Corporation Tax with the right reference

Pay by the payment date with the period's 17-character reference, found on the notice, HMRC's reminders and the company's HMRC online account; a wrong reference can pay a different bill (GOV.UK: Pay Corporation Tax). If nothing is due, tell HMRC with its "nil to pay" form (GOV.UK: No payment due).

Step 5: Prepare and file the Company Tax Return (CT600)

File online with commercial software that can file the CT600, the tax computation and the accounts; GOV.UK allows paper only with a reasonable excuse or in Welsh (GOV.UK: HMRC filing service closure). The return includes a declaration that it is correct and complete, so review every figure and get advice where you're unsure.

Step 6: Check that HMRC accepted the return and the payment

Check that HMRC accepted the return, not just that it was sent, and keep the acceptance. If HMRC rejects it, fix the error and resend before the filing date. Then check the payment, which GOV.UK says shows in the company's HMRC online account within a few days (GOV.UK: Check your payment).

What should you do if the company was dormant or the CT603 dates are wrong?

File the return anyway if the company was dormant: GOV.UK says a company that has had a notice to deliver must still file a Company Tax Return online, which shows HMRC it was dormant for that period (GOV.UK: Dormant for Corporation Tax). If the dates on the notice are wrong, tell HMRC through its Corporation Tax enquiries service.

A notice usually means HMRC's records show the company as active, because HMRC's manual says a notice to deliver isn't issued to a dormant company (HMRC manual COM130130). The company can also tell HMRC it is dormant using its UTR; after that it files no further return unless HMRC asks or it starts doing business again (GOV.UK: Tell HMRC your company is dormant). For dates, a company that lengthened its year must contact HMRC before the original filing date, and one with accounts shorter than 12 months enters the new dates in its software before filing (GOV.UK: Accounting periods). Our dormant company tax return guide covers the nil return.

What happens if you ignore a CT603 or file late?

A late return costs £200, plus £200 more at 3 months, for filing dates from 1 April 2026. Then come 10% of the tax still unpaid if the return is 6 months late (18 months after the accounting period ends), rising to 20% if it is 12 months late (2 years after the period ends) (GOV.UK: late filing).

The flat amounts are set in law: £200 if the return arrives within 3 months after the filing date and £400 after that (FA 1998 Sch 18 para 17), up from £100 and £200 for filing dates before 1 April 2026 (GOV.UK: penalty increases). A return late 3 times in a row raises the £200 penalties to £1,000 each (GOV.UK: late filing). The tax-geared penalty has its own trigger: a return not delivered within 18 months after the period ends, or by the filing date if that is later, as it can be after a late notice (FA 1998 Sch 18 para 18).

At 6 months late, HMRC also sends a tax determination of the tax it thinks the company owes; it can't be appealed, and the company must pay and file (GOV.UK: late filing). A penalty can be appealed with a reasonable excuse once the return is filed, usually within 30 days of it being issued (GOV.UK: Disagree with a penalty). See filing a CT600 late and which overdue CT600 to file first. Taxley's free penalty calculator assumes the notice was on time.

Is the payment deadline different from the CT603 filing date?

Yes. Corporation Tax is due 9 months and 1 day after the end of the accounting period for a company with taxable profits up to £1.5 million, about 3 months before the return is due (GOV.UK: Pay Corporation Tax). A late notice can move the filing date, but the tax stays due on the same day.

The payment date comes from a different law: Corporation Tax is due on the day following the expiry of 9 months from the end of the accounting period (TMA 1970 s.59D), with no reference to any notice. HMRC may charge interest on tax paid late (GOV.UK: Pay Corporation Tax). Late filing and late payment are charged separately, so a company that can't pay on time should still file by its filing date. Our guides to Corporation Tax deadlines and how to pay Corporation Tax cover the rest.

How can Taxley help you answer a CT603?

Taxley (taxley.co.uk), UK online software that prepares and files the Company Tax Return (CT600) with HMRC and the annual accounts with Companies House, handles the filing side of a CT603. It prepares the return, the tax computation and the accounts, then files the CT600 using the company's own Government Gateway user ID.

Taxley keeps the user ID encrypted and never keeps the password. Once you've started a return, the dashboard's "Upcoming deadlines" list shows "Pay by" and "File by" dates for each company, with an "Add to calendar" link. The dashboard counts the filing date as 12 months after the period ends and doesn't use the notice date, so after a late notice it shows a date earlier than the company's actual filing date. Accounts longer than 12 months are split into two returns, sent together. The Dormant option prepares a nil return for £14.95. When you amend a return filed with Taxley, you can enter the date of a notice HMRC sent more than 9 months after the period ended, and Taxley works out the time to amend from it.

The fee is £49.95 with micro-entity accounts or £129 with small-company accounts, including filing the accounts at Companies House once the return is paid and the accounts are finished, with the company's 6-character authentication code. Taxley doesn't pay the company's Corporation Tax, tell HMRC the company is dormant, update its accounting period dates with HMRC or file the confirmation statement. Take the 30-second check to see whether Taxley fits your company, or see the prices.

Frequently asked questions

What if the company is trading but never received a CT603?

It must still act. A chargeable company without a notice must tell HMRC within 12 months of the period end (FA 1998 Sch 18 para 2). HMRC treats a return sent without a notice as if a notice was served the day it arrived (HMRC manual CTM93050).

Does an accountant get a copy of the CT603?

No. HMRC's manual says agents don't receive copies of notices to deliver; HMRC sends them monthly lists of the clients it has issued notices to instead (HMRC manual COM130130). If an accountant files for the company, pass the CT603 on when it arrives.

Can I send a paper return in reply to a CT603?

Only with a reasonable excuse or to file in Welsh; otherwise GOV.UK says you must file online. HMRC's own online filing service closed on 31 March 2026, so filing online now means commercial software (GOV.UK: HMRC filing service closure).

Does getting a CT603 mean the company owes Corporation Tax?

No. A notice asks for a return, not a payment: the company must file even with a loss or no tax to pay (GOV.UK: Company Tax Returns), and a late return still gets the flat late-filing penalties when no tax is due.

General information, not personalised tax or accounting advice.