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AFTER PREPARATION, BEFORE ACCEPTANCE

CT600 submission to HMRC, explained

What actually travels to HMRC when you submit a Company Tax Return? Follow the electronic package from transmission and IRmark receipt through HMRC's later checks—and learn what to do if a return is rejected or needs amending.

Start your filing Pre-submission checklist

01 / THE PACKAGE

What is sent to HMRC?

A CT600 submission is one electronic package: CT600 Version 3 return data in XML, iXBRL-tagged statutory accounts and an iXBRL tax computation, with supplementary pages where relevant. Those may include CT600A for loans to participators or CT600E for charities and CASCs. Applicable accounts-attachment arrangements differ for some entities and amendments.

Commercial software sends the return using the company's own Corporation Tax-enrolled Government Gateway user ID and password. Filing accounts with Companies House is a separate submission with its own authentication code, deadline and result—not part of HMRC's acknowledgement.

New to the return itself? Start with the CT600 preparation guide or the canonical introduction to the Company Tax Return.

02 / TWO STAGES

Receipt is not the same as posting

Gateway acknowledgement

HMRC's online gateway checks the submitted format, credentials and iXBRL, then returns an electronic response— typically within seconds to a couple of minutes. A successful first-stage response includes an IRmark receipt: a digital fingerprint that is HMRC's proof of receipt. Keep the receipt and check the response for each return submitted.

Corporation Tax back-office checks

HMRC's back-office then posts the return to the company's record. Occasionally this can fail even after a successful gateway response and IRmark—for example, if the accounting-period dates differ from HMRC's records, the UTR is wrong, the UTR and company number do not match, or HMRC has no notice to deliver for that period. HMRC may notify the taxpayer through the email address on the Gateway account or by letter.

If HMRC reports a later failure

Do not blindly resubmit the same return. Obtain HMRC's correct accounting-period dates in writing first, then resolve the discrepancy before deciding how to refile.

03 / AVAILABLE ROUTES

Ways to submit a CT600 in 2026

HMRC's former free joint online filing service (CATO) closed on 31 March 2026. Online returns now normally use commercial software. Paper filing is permitted only in limited circumstances, such as where HMRC accepts a reasonable excuse for not filing online or a return is filed in Welsh; check GOV.UK's current Company Tax Return guidance.

Ways to submit a CT600 in 2026
QuestionWeFileFormer HMRC free service (CATO)Paper return
Still available?Yes, for supported filings.No. Closed 31 March 2026.Only in limited permitted cases; check GOV.UK.
Prepares iXBRL accounts and computation?Yes, for supported accounts and tax circumstances.Previously supported eligible companies within its limited scope.No automatic preparation; provide applicable supporting documents.
Checks P&L and balance sheet reconcile?Yes, within the guided workflow.Not a currently available workflow.No software reconciliation check.
Calculates Corporation Tax, including Marginal Relief?Yes, for supported circumstances.Not a currently available calculation route.Calculate and check the figures separately.
Supplementary pages?CT600A and CT600E where applicable; not every page is supported.Support was limited.Include applicable pages under HMRC's paper-filing rules.
Companies House accounts in the same workflow?Yes, for supported companies; the submission is separate and its status is tracked separately.Joint filing was available for some companies while the service ran.File with Companies House separately where required.
Submission status and IRmark receipt shown?Yes, HMRC responses and the IRmark appear in submission history when available.No longer available for new submissions.No electronic IRmark through a paper submission.
Cost?
Free while it operated.No software fee; other preparation costs may apply.

See the guide to filing after the free service closed or the date-qualified software comparison for more on choosing a route.

04 / BEFORE TRANSMISSION

How WeFile prepares the return

Import a trial balance from Xero, QuickBooks Online, FreeAgent or CSV, or enter your figures yourself. The workflow checks that the profit and loss account and balance sheet reconcile, calculates the supported Corporation Tax position and prepares accounts under the applicable supported regime: FRS 105 micro-entity or FRS 102 Section 1A/full.

Review the generated accounts, tax computation and CT600 Version 3 before submitting. WeFile tracks the HMRC response and displays an IRmark in submission history when HMRC supplies one. An accounts period longer than 12 months is split into two CT600 returns, because a single Corporation Tax accounting period cannot exceed 12 months.

For the screen-by-screen preparation process, see the online CT600 filing walkthrough. Reconciliation checks and an electronic receipt do not guarantee HMRC's eventual acceptance or posting.

05 / IF SOMETHING FAILS

Why a submission may be rejected

Authentication failureWrong Government Gateway credentials or an account not enrolled for Corporation Tax. The return was not submitted; correct the credentials before trying again.
Incorrect identifiersA wrong company UTR or a UTR that does not match the company number needs investigation before resubmission.
Period mismatch or duplicateCheck the period HMRC holds. If an original return was already received for the same period, an amendment may be needed instead.
iXBRL or validation errorsReview the reported validation issue, supporting documents and return figures before submitting again.

Read the guide to HMRC CT600 submission errors for further troubleshooting. If HMRC reports a failure after an IRmark receipt, first obtain its period dates in writing rather than repeatedly sending the same return.

06 / KEEP THE DATES SEPARATE

Submission and payment deadlines

12 monthsA CT600 is normally due 12 months after its Corporation Tax accounting period ends.
9 months + 1 dayCorporation Tax is normally payable by this point after period end for companies outside the instalment-payment regime.

For returns with a filing date on or after 1 April 2026, HMRC's fixed late-filing penalties are £200 when the return is one day late and a further £200 when it is more than three months late (£400 in fixed penalties overall). For three successive late returns, those amounts rise to £1,000 and a further £1,000 (£2,000 overall). Additional tax-geared penalties of 10% of unpaid tax at 18 and 24 months may apply.

Sources: GOV.UK penalty increase, GOV.UK late-return rules, and the Corporation Tax deadlines and penalties guide. Companies House accounts have a separate deadline and penalty regime.

07 / CORRECTING A RETURN

What if you need to amend it?

A Company Tax Return can usually be amended within 12 months of its filing deadline. An amendment may revise only the CT return or tax computation, leaving the statutory accounts unchanged—in that case, there is nothing new to file with Companies House. If the statutory accounts also need revising, their filing is a separate step.

Check GOV.UK guidance on changing a Company Tax Return and seek qualified advice if the correction or deadline is uncertain.

QUESTIONS, ANSWERED

CT600 submission frequently asked questions

HMRC's free joint online filing service closed on 31 March 2026. Online Company Tax Returns now normally use commercial software and the company's Corporation Tax-enrolled Government Gateway credentials. Paper filing is allowed only in limited circumstances, including an accepted reasonable excuse for not filing online or filing in Welsh.
You generally need CT600 Version 3 return data, statutory accounts and a tax computation in iXBRL, plus any applicable supplementary pages, your company UTR, period dates and Corporation Tax-enrolled Government Gateway credentials. Some entities and amendments have different accounts-attachment arrangements. Filing accounts with Companies House is a separate submission.
Check HMRC's electronic response and keep the IRmark receipt included in a successful first-stage response. The IRmark is HMRC's proof of receipt. It does not guarantee that the Corporation Tax back-office has posted the return to the company's record; HMRC can report a later failure.
Possible reasons include incorrect or unenrolled Government Gateway credentials, a wrong UTR, a UTR and company-number mismatch, accounting-period dates that do not match HMRC's records, a return already received for the period, or iXBRL validation errors. Fix the reported issue before trying again; an authentication failure is not a submitted return.
Yes. A director may prepare and submit a Company Tax Return using suitable software without appointing an accountant, but remains responsible for accuracy, completeness and deadlines. Consider qualified advice for complex circumstances.
Yes. WeFile supports FRS 105 micro-entity accounts and FRS 102 Section 1A and full accounts for applicable filings. FRS 105 does not permit fair value accounting; select the accounts regime appropriate to your company.
For returns with a filing date on or after 1 April 2026, HMRC's fixed penalty is £200 when one day late and a further £200 when more than three months late (£400 overall). For three successive late returns, these rise to £1,000 and a further £1,000 (£2,000 overall). Tax-geared penalties may also apply at 18 and 24 months. Corporation Tax payment has a separate deadline.

This page and checklist are general information, not professional accounting or tax advice. Directors remain responsible for accurate, complete and timely filings. An electronic receipt does not guarantee that HMRC's back-office checks have posted a return to the company's record.