The CT603 notice to deliver a Company Tax Return

By Max Szolcek, founder of Dormant Return. Last updated 3 September 2026

A Company Tax Return is not due because a year has passed, or because the company made money. It is due because HMRC has asked for one. The asking is done by a letter called a notice to deliver a Company Tax Return, known by its form number, CT603. This guide explains what the notice is, what it sets in motion, and what to do when one lands on the doormat of a company that has done nothing all year.

What the notice is

Gov.uk puts the rule in one sentence: your company must file a Company Tax Return if you get a "notice to deliver a Company Tax Return" from HMRC. The notice is the legal trigger. Without it, no return is required for that period. With it, a return is required whether the company traded, made a loss, or sat untouched with nothing in the bank. Gov.uk is equally plain on that point: you must still send a return if you make a loss or have no Corporation Tax to pay.

The notice is a different letter from the one a new company receives shortly after incorporation. That first letter gives the company its 10-digit Unique Taxpayer Reference and explains how to register the company as active with HMRC. A CT603 comes later, once HMRC believes the company has an accounting period for which it wants a return, and then normally once a year for as long as HMRC expects the company to keep filing.

What it tells you

The notice identifies the company and its Corporation Tax reference, and specifies the period HMRC wants a return for. That period is the important part. HMRC's own guidance for its staff explains that a company must work out which period, or periods, the notice requires a return for, because the period HMRC specifies does not always line up with the company's own accounting periods. If your company's accounts cover more than twelve months, for instance, one notice can mean two returns; see why HMRC expects two CT600s for a first period.

If you have lost the letter, the reference and the period are also visible in the company's HMRC business tax account, and the reference can be recovered separately; see how to find your company's Corporation Tax UTR.

The deadline it sets

The general rule, in gov.uk's words, is that the deadline for your tax return is 12 months after the end of the accounting period it covers. HMRC's Company Taxation Manual adds the rule that matters when a notice arrives late: the filing date is the later of twelve months from the end of the period and three months from the date when the notice requiring the return is served. So a notice issued eleven months after a period ended still leaves you three months to file, not one.

Two separate dates are easy to confuse. The return deadline is twelve months after the period ends. The deadline to pay any Corporation Tax is usually nine months and one day after the period ends. A dormant company owes nothing, so only the first date matters to it. The CT600 deadline calculator works both out from an accounting period end date.

What if the period on the notice is wrong, or the company never traded?

A notice is not a verdict that the company was active. HMRC issues them on the information it holds, and for a company that has never done anything that information is often just the incorporation date. If the company is genuinely dormant, the answer is to tell HMRC so. Gov.uk states that once you have told HMRC your company is dormant, you do not need to pay Corporation Tax or file another Company Tax Return, unless you receive a further notice to deliver one. How to do that, and what HMRC sends back, is in how to tell HMRC your company is dormant.

Be careful with the word "another" in that sentence. Telling HMRC the company is dormant stops future notices. It does not by itself cancel a notice that has already been issued for a period that has already ended. If you hold a CT603 for a completed period, the safe course is to file the return it asks for, which for a dormant company is a CT600 of zeroes with dormant accounts attached, and to tell HMRC about the dormancy at the same time so the next notice never comes. If the period shown is plainly wrong, contact HMRC before the deadline, not after it.

Holding a CT603 for a dormant company? Dormant Return prepares the CT600 and iXBRL dormant accounts the notice requires and files them to HMRC under your own Government Gateway sign-in. About five minutes, £5, refunded automatically if HMRC rejects the return.

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What happens if you ignore it

Penalties for a late Company Tax Return are fixed and automatic, and they do not depend on any tax being due. Gov.uk's table starts at £200 one day after the deadline, another £200 three months later, then percentages of unpaid tax at six and twelve months. If your tax return is late three times in a row, the £200 penalties are increased to £1,000 each. For a company with nothing to pay, the percentage penalties are nil but the flat ones are not, which is how a company that has never earned a penny can build up a four-figure bill. The full picture is in late CT600 penalties for dormant companies.

The one-line summary for dormant companies

If HMRC has sent your dormant company a CT603, a CT600 is due for the period it names, and no amount of nil figures changes that. If HMRC has agreed the company is dormant and has not sent one, nothing is due until it does. The reasoning behind both halves is set out in do dormant companies need to file a CT600?