Key facts
- All Company Tax Returns (CT600) must normally be filed electronically — paper returns are accepted only with a reasonable excuse or when filing in Welsh.
- Statutory accounts and tax computations must be in iXBRL (inline eXtensible Business Reporting Language) format.
- iXBRL “tags” embed machine-readable data within a human-readable document.
- HMRC’s free filing service (CATO) and its joint filing option closed on 31 March 2026 — commercial software is now required.
How iXBRL Digital Filing Works for Company Accounts
iXBRL stands for inline eXtensible Business Reporting Language. It is a data format that combines a human-readable document (which looks like a normal set of accounts) with machine-readable tags embedded in the HTML code.[1]
When you open an iXBRL file in a web browser, it looks like a normal financial statement. But hidden within the code are structured data tags that tell HMRC’s systems exactly what each number means — for example, that a particular figure is “revenue,” “cost of sales,” or “tax payable.”
HMRC requires iXBRL so it can:
- Automatically process millions of tax returns without manual data entry
- Cross-check figures between the accounts, computation, and CT600 return
- Analyse trends and identify potential compliance risks across all companies
What Must Be Filed in iXBRL?
When submitting a CT600, you must include two iXBRL documents:[1]
| Document | Taxonomy | Description |
|---|---|---|
| Statutory accounts | UK GAAP / FRS 102 / FRS 105 taxonomy | The company’s annual financial statements tagged to the relevant accounting standard |
| Tax computation | Corporation Tax computation taxonomy | The calculation showing how accounting profit is adjusted to taxable profit |
The CT600 form itself is a structured data submission (not iXBRL) — it is the accounts and computation attachments that must be in iXBRL format.[4] HMRC-recognised software such as GoFile’s Corporation Tax filing tool generates the iXBRL attachments and submits everything together.
Understanding Taxonomies
A taxonomy is a standardised dictionary of tags. HMRC publishes specific taxonomies that define which data items can be tagged and how they relate to each other.[4]
The main taxonomies used are:
- UK-GAAP (FRS 102) — for companies preparing accounts under UK GAAP
- UK-GAAP (FRS 105) — for micro-entities using the simplified standard
- EU-IFRS — for companies preparing accounts under International Financial Reporting Standards
- DPL (Detailed Profit & Loss) — additional detailed tags for the tax computation
- CT computation — tags for the Corporation Tax calculation itself
Minimum tagging: HMRC requires a minimum set of items to be tagged — you cannot submit an empty or sparsely-tagged document. The exact minimum depends on the accounting framework used and the company’s size.[4]
Software Options
You do not need to manually create iXBRL files. Several categories of software handle the tagging automatically:[2]
| Option | Best For | Examples |
|---|---|---|
| Integrated accounting software | Companies preparing accounts and tax return in one place | Xero, Sage, QuickBooks (with CT add-ons) |
| Specialist CT software | Accountants filing on behalf of clients | CCH, Alphatax, Digita, TaxCalc |
| iXBRL tagging tools | Companies with accounts in another format that need iXBRL conversion | Arkk Solutions, CoreFiling, ParsePort |
| HMRC free filing service (CATO) | Closed 31 March 2026 | No longer available — companies that used it now need commercial software |
Filing with HMRC and Companies House
HMRC’s free service previously offered joint filing — submitting statutory accounts to HMRC and Companies House in one go. That service closed on 31 March 2026, so the two filings are now made separately:[2]
- HMRC — iXBRL accounts and tax computation filed with the CT600 through commercial software
- Companies House — statutory accounts filed through its own online service or via software; Companies House is itself moving to software-only accounts filing from 1 April 2027
Tip: The accounts filed at Companies House may be abridged (if the company qualifies as small or micro), while full accounts go to HMRC with the CT600. The deadlines also differ: Companies House accounts are due 9 months after year end, the CT600 12 months after the accounting period ends.
Common Reasons for Rejection
HMRC’s systems will reject a CT600 submission if:
- The accounts or computation are not in iXBRL format (e.g. submitted as a plain PDF)
- The iXBRL file contains validation errors (e.g. tags that don’t match the published taxonomy)
- Required mandatory tags are missing (e.g. company name, period dates, or key financial figures)
- The accounting period dates in the iXBRL file don’t match those on the CT600
If a submission is rejected, it does not count as filed. You must correct the errors and resubmit before the filing deadline to avoid penalties.
Frequently Asked Questions
What is iXBRL?
iXBRL stands for inline eXtensible Business Reporting Language. It is a data format that combines a human-readable document (like a normal set of accounts) with machine-readable tags embedded in the HTML code, allowing HMRC to process returns automatically.
Do I need to create iXBRL files manually?
No. Commercial software handles iXBRL tagging automatically. HMRC’s free filing service, which previously covered simple company structures, closed on 31 March 2026 — companies now use HMRC-recognised commercial software instead.
Can I submit accounts as a PDF with my CT600?
No. HMRC requires statutory accounts and tax computations in iXBRL format. A plain PDF submission will be rejected, and the return will not count as filed.
What is joint filing?
Joint filing let companies submit statutory accounts to HMRC and Companies House in a single submission through HMRC’s free online service. That service closed on 31 March 2026, so accounts are now filed with HMRC (with the CT600, via software) and separately with Companies House.
Further Reading
- The CT600 Tax Return — what goes into the Company Tax Return
- Company Accounts Requirements — the different types of statutory accounts
- Filing Deadlines — when your return and accounts are due
- Amending a CT600 — how to correct mistakes after filing
Looking for simple Corporation Tax software?
#GoFile is HMRC-recognised and trusted by 50,000+ UK businesses. Set up in minutes, file with confidence.
Get Started For FreeNo credit card required · Cancel anytime