How long does it take to file a CT600? What you can still do before the deadline

In short: A Company Tax Return (CT600) goes to HMRC online, and HMRC sends an acknowledgment of receipt when you submit it (GOV.UK: Company Tax Return obligations), so a company with its figures and a working HMRC sign-in can file on deadline day. What can't be rushed is the post: HMRC's activation code takes up to 10 days (GOV.UK: Add Corporation Tax services). Taxley prepares and files the CT600 for £49.95 with micro-entity accounts, and files the accounts at Companies House once the return is paid and the accounts are finished.

Take the 30-second check to see whether Taxley fits your company, or get your own dates from the Corporation Tax deadline calculator.

Key facts (checked on 10 October 2026)

How long does it take to file a CT600?

The online submission itself is quick: HMRC acknowledges receipt when you submit. The real time goes on two things: preparing the accounts, computation and return, which depends on your records, and any code or reference that only comes by post. If the accounts and computation are ready and the sign-in works, the return can be filed the same day.

The deadline doesn't stretch to fit. By law the filing date is the last day of whichever of several periods ends latest, including 12 months from the end of the return period and 3 months from the date HMRC served its notice to deliver (FA 1998 Sch 18 para 14). For most companies that means 12 months after the accounting period ends (GOV.UK: Company Tax Returns), so a return filed on the last day is on time. HMRC's acknowledgment confirms receipt only, not that HMRC agrees the figures; HMRC can still correct obvious errors or open an enquiry (GOV.UK: Company Tax Return obligations). The Corporation Tax itself is due earlier, usually 9 months and 1 day after the period ends (GOV.UK: Pay your Corporation Tax bill). What a return must contain is covered in what you need to file a CT600, and the filing steps in how to file a CT600 yourself.

Which parts of filing a CT600 can't be rushed?

Anything that arrives by post or moves through a payment system. HMRC posts the Corporation Tax activation code within 10 days, or 21 if you live abroad, and Companies House posts its authentication code within up to 10 working days. Bacs payments usually take 3 working days, and a first Direct Debit 5.

HMRC's activation code goes by post to the company's registered office, where all its Corporation Tax letters are sent (GOV.UK: Add Corporation Tax services). If no UTR arrives 15 working days after registering, you can ask for it online, and HMRC sends it to the business address registered with Companies House; GOV.UK gives no delivery time for that copy (GOV.UK: Find your UTR number). Companies House warns not to wait until the accounts are due to request its code, because delivery can take longer in busy periods, and it won't email the code or read it out by phone (GOV.UK: Company authentication codes). On the day itself, entering the wrong sign-in details too many times locks the HMRC account for 2 hours, which helpdesk advisers can't undo (GOV.UK: problems signing in).

Can you still file a CT600 if it's due tomorrow?

Yes, if the company's Government Gateway account already has Corporation Tax added and you know the user ID and password, the UTR and the figures. The return goes online and HMRC acknowledges receipt on submission. Without a working sign-in, a new activation code takes up to 10 days, so the deadline will probably pass.

With a day left, use it in a set order. First check the sign-in: sign in to the company's business tax account, which shows the taxes it is registered for, and confirm Corporation Tax is one of them (GOV.UK: Sign in to your HMRC business tax account). The company's user ID is not a director's personal one (GOV.UK: Set up a private limited company). Then prepare the accounts and computation, which must go in iXBRL through commercial software (GOV.UK: Company Tax Return obligations), and file early in the day. GOV.UK says online services may be slow at busy times (GOV.UK: problems signing in), and a rejected return still has to be corrected and resent before the deadline passes. If HMRC rejects the sign-in with error 1046, our guide to error 1046 explains the fixes.

What can you still do with 6 weeks, 3 weeks or 1 day left?

The more days left, the more postal steps still fit. With 42 days or more, any code can be requested and, if lost, requested again, even from abroad. Below 21 days, a code posted abroad may not arrive in time, and below 10 days only what you already hold counts. The table sets out each stage.

The windows come straight from GOV.UK's stated times: up to 10 days for the activation code, or 21 abroad (GOV.UK: Add Corporation Tax services), and up to 10 working days for the Companies House code (GOV.UK: Company authentication codes). The bands are a planning aid, not HMRC rules. A lost or expired letter means a second wait of the same length, so the 42-day line allows two 21-day waits from abroad, and the 21-day line two 10-day waits in the UK. Once the deadline has passed, the return still has to be filed, and filing quickly keeps the company inside the first penalty band. Our guide to the activation code adds spare days for a lost letter.

What is the latest day to request each code?

For a 12-month period ending 31 January 2026 that isn't the company's first, the Companies House code must be requested by Friday 16 October 2026 and the HMRC activation code by Thursday 21 January 2027, if each letter takes its full stated time. Those are last possible days, with nothing spare.

Worked timeline: year ended 31 January 2026 (not a first period; England and Wales working days)

The deadlines follow GOV.UK: accounts 9 months after the financial year ends, tax 9 months and 1 day after the accounting period, and the return 12 months after (GOV.UK: Accounts and tax returns for private limited companies). The tax date falls on a Sunday, and GOV.UK says a payment must then reach HMRC on the last working day before, Friday 30 October, unless you pay by Faster Payments, which usually arrive the same or next day, weekends included (GOV.UK: Pay your Corporation Tax bill). Counting back 10 working days from Friday 30 October gives Friday 16 October, with no bank holidays in between (GOV.UK: Bank holidays). The activation code dates count calendar days back from 31 January 2027. Each date leaves no time to replace a lost letter, so treat it as the last moment, not the plan.

What happens if you miss the CT600 deadline by a day?

HMRC charges a £200 penalty as soon as the return is one day late, for returns whose filing date is on or after 1 April 2026, even if there's no tax to pay. A second £200 follows at 3 months late, and larger tax-geared penalties start at 6 months.

For returns due from 1 April 2026, the penalties are £200 at 1 day late and £200 more at 3 months (GOV.UK: increases to late filing penalties), then 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: Company Tax Returns). If a return is late 3 times in a row, the £200 penalties become £1,000 each. Penalties apply even when no tax is due (GOV.UK: Pay your Corporation Tax bill). In the worked timeline, filing on Monday 1 February 2027 costs £200, and filing after Friday 30 April 2027 costs £400. You can appeal with a reasonable excuse, but only after filing the return (GOV.UK: Company Tax Returns). Our guide to filing a CT600 late covers appeals and determinations.

Why do the Companies House accounts usually fall due first?

Because a private company's annual accounts are due at Companies House 9 months after its financial year ends, 3 months before the CT600. For most companies the accounts deadline is the one that arrives first, and filing them needs the authentication code, which comes by post.

First accounts follow a different rule: they are due 21 months after the company registered with Companies House (GOV.UK: Accounts and tax returns for private limited companies). The two filings can go together or separately, so the accounts don't have to wait for the CT600. Late accounts cost a private company £150 for up to 1 month, rising to £1,500 after 6 months, and the penalty doubles if accounts are late 2 years in a row (GOV.UK: Prepare annual accounts). You can apply to extend the accounts deadline only before it passes, and only for an event outside your control, such as a fire that destroyed the records (GOV.UK: extend your accounts filing deadline). Companies House aims to process most online filings within 24 hours and sends an acknowledgement almost immediately (GOV.UK: Filing online). Our guide to a lost authentication code covers a code that never arrived.

How does Taxley help when the CT600 deadline is close?

Taxley helps with the preparation, not the post. It looks up the company at Companies House, offers the periods Companies House holds, imports a trial balance, builds the iXBRL accounts and computation, and files with the company's own Government Gateway sign-in. It can't make HMRC's or Companies House's letters arrive sooner.

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, starts with a Companies House look-up by name or number that fills in the company name and registration number, then loads the periods Companies House holds; a period can only be filed once it has ended. You can import a trial balance from bookkeeping software as a CSV or Excel file, or paste the rows, and check where each account goes before it is applied. Simple mode puts straightforward micro-entity accounts and tax on one page. Taxley builds the accounts and computation in iXBRL and checks their format before you file. The dashboard lists each company's upcoming deadline, with an "Add to calendar" link, and the return shows the tax to pay and its pay-by date.

To file, you confirm the declaration and pay one fee: £49.95 with micro-entity accounts, £139 with small-company accounts, or £9.50 for a dormant company's nil return. You then enter the company's own Government Gateway user ID and password; Taxley never keeps the password. The filing page shows "Accepted by HMRC" with the date and time, and if HMRC rejects the return, correcting it and sending it again costs nothing extra, though the deadline still applies. With the authentication code saved, Taxley files the accounts at Companies House once the return is paid and the accounts are finished. It doesn't pay the company's Corporation Tax, doesn't file the confirmation statement and isn't tax advice, so review your figures before you file. Take the 30-second check to see whether Taxley fits your company, or see the price for your company.

Frequently asked questions

Does a weekend deadline change when the tax must be paid?

Yes. If the payment deadline falls on a weekend or bank holiday, GOV.UK says the payment must reach HMRC on the last working day before it, unless you pay by Faster Payments (GOV.UK: Pay your Corporation Tax bill). A card payment counts on the date you make it.

How long does HMRC take to acknowledge an online CT600?

GOV.UK says you receive an acknowledgment of receipt when you submit the return online (GOV.UK: Company Tax Return obligations). It gives no waiting time. The acknowledgment doesn't mean HMRC has agreed the figures, and HMRC can still correct errors or enquire.

Can I file the CT600 before the accounts are at Companies House?

Yes. GOV.UK says a private limited company can file with Companies House and HMRC together or separately (GOV.UK: Accounts and tax returns for private limited companies). Each has its own deadline, and in a normal year the accounts fall due 3 months before the return.

Is there a late-filing penalty if the company owes no tax?

Yes. GOV.UK says late filing penalties are charged if the Company Tax Return isn't filed on time, even if there is no tax to pay (GOV.UK: Pay your Corporation Tax bill). A company that made a loss must still send its return by the deadline.

General information, not personalised tax or accounting advice.