If you have been searching for “new CT600 regulations”, you will find two stories mixed together. One is true: the way you send a Company Tax Return to HMRC changed in 2026. The other is not: Making Tax Digital for Corporation Tax is not a live mandate, and HMRC has said it does not intend to introduce it in the form originally consulted on.
What did not change
- You still file a CT600 for each Corporation Tax accounting period (no longer than 12 months).
- You still pay Corporation Tax nine months and one day after the period ends (for most companies).
- You still file the return within 12 months of the period end.
- There are no quarterly MTD-style updates for Corporation Tax.
What did change: the free filing route closed
On 31 March 2026 HMRC closed the free online service that many small companies used to file accounts and a Company Tax Return together. From 1 April 2026, sending a CT600 generally means using commercial software that can talk to HMRC’s Corporation Tax Online service (XML / iXBRL), unless a rare paper exception applies.
A complete software submission is still the familiar package: the CT600 return, a Corporation Tax computation in iXBRL, and accounts in iXBRL where required. Companies House accounts remain a separate filing with their own routes and fees.
Rates, associated companies and computations still matter
The filing channel changed; the tax calculation did not go away. Companies still need the right period dates, the small profits and main rates where they apply, associated-company counts, capital allowances (including full expensing where available), and any R&D or other reliefs on the correct supplementary pages. HMRC will still query a return that does not hang together.
Identity, agents and who can file
Directors must keep Companies House identity verification in order. That does not replace the CT600, but a practice that files both corporation tax and confirmation statements needs the client’s personal codes and HMRC credentials in one place. Agents filing for clients should keep Government Gateway / agent services organised before the deadline, not on it.
HydraTax prepares and files CT600 packages through HMRC-ready software so you are not dependent on a closed GOV.UK form. Always check the latest GOV.UK guidance for rates and reliefs before you submit.
A short checklist for the next CT600
- Confirm the accounting period dates with Companies House and HMRC.
- Lock a trial balance or tagged bank year-end so the iXBRL accounts and CT computation use the same figures.
- Use commercial CT software — not a screenshot of last year’s PDF.
- Keep the computation, accounts and CT600 together as one submission.
- Diary the payment date separately from the filing date; they are not the same.
HMRC may still modernise corporation-tax administration in future. Until it publishes a new regime with dates, treat the annual CT600 as the live obligation — and treat software filing as the live method.