Will I be fined for filing late because of this?

The filing deadline for a Company Tax Return is twelve months after the end of your accounting period, so for most affected companies there is still a considerable amount of time. We are not able to tell you what view HMRC will take if the delay runs on, and we would not want to give you a reassurance that is not ours to give — if your deadline is approaching, please ask HMRC directly on 0300 200 3410.

Should I wait, or use another provider?

Another provider will not help. HMRC publishes a single taxonomy that all commercial software must use, and it is the same missing file for everyone.

Will I have to enter everything again?

No. Your figures are saved against your filing and will still be there. When the taxonomy is published you will be able to carry on from where you stopped.

Can I still file my accounts at Companies House?

Yes. Accounts use a different taxonomy, published by the Financial Reporting Council, and it is available. Your Companies House deadline is unaffected and should be met as normal.

Does this delay when I have to pay?

No. Corporation Tax is due nine months and one day after your period ends, whether or not the return has been filed, and HMRC charges interest on late payment. Pay by the normal date if you can.

Periods ending after 31 March 2026: why your return cannot be filed yet

If your accounting period ends on or after 1 April 2026, your Company Tax Return cannot be filed yet — by us or by anyone else. The reason is a missing file at HMRC rather than anything to do with your company, and there are still things worth doing in the meantime.

What has happened

A Company Tax Return is not a simple form. The accounts and the tax computation must both be tagged in iXBRL, a machine-readable format, and the tags come from a published dictionary called a taxonomy. Software cannot invent one: HMRC publishes the file, and every return is checked against it.

HMRC’s published list of accepted taxonomies shows Corporation Tax computational 2024 as valid for periods ending up to 31 March 2026. Its replacement, Corporation Tax computational 2025, is named on the same page with no end date — but the file itself has not been released, and no download link is given.

The result is a gap. For an accounting period ending on or after 1 April 2026, the taxonomy you are required to use does not yet exist to download.

Who is affected

Any company whose accounting period ends on or after 1 April 2026. A company with a 31 March year end is unaffected; one with a 30 April year end is affected.

This is not specific to OneFiler. HMRC publishes one taxonomy for every provider, so no commercial software can produce a compliant computation for these periods until the file is released. Filing on paper is not an alternative either — HMRC requires online filing for Corporation Tax.

Your Companies House accounts are not affected

Accounts are tagged against a different dictionary, published by the Financial Reporting Council rather than HMRC, and that one is available. Your annual accounts can therefore be filed at Companies House now, on time, in the normal way.

It is worth doing. The Companies House deadline usually falls well before the HMRC one — nine months after your period end against twelve — so there is no reason to let a Corporation Tax delay put your accounts at risk.

Paying your Corporation Tax is separate, and still due

This is the point most easily missed. Payment and filing are two different obligations with two different deadlines. Corporation Tax is payable nine months and one day after the end of your accounting period, which is three months before the return itself is due.

A delay in filing does not postpone the payment date, and HMRC charges interest on tax paid late. If you know roughly what you owe, pay it by the normal date and settle any difference once the return goes in.

What to do now

Build your return as usual. Everything up to the final step works, your figures are saved, and nothing will need re-entering when the taxonomy arrives.

File your accounts at Companies House to meet that deadline.

Pay your Corporation Tax by the normal date if it falls before the taxonomy is published.

If your filing deadline is close and you are concerned, HMRC’s Corporation Tax helpline is 0300 200 3410. They can tell you where you stand — we would rather you heard that from them than from us.

What we are doing

We have written to HMRC’s Software Developers Support Team asking when the Corporation Tax computational 2025 taxonomy will be published, and we check the published list regularly.

Support will be added as soon as the file is released and we have validated a return against HMRC’s own test service. We will not guess at the format: emitting a taxonomy reference we cannot validate would produce returns HMRC rejects, which is worse than waiting.