How to file a CT600 Company Tax Return and accounts online
Why HMRC closed its free online CT600 filing portal, iXBRL company accounts rules, and how to file your UK company accounts and CT600 online for a fixed fee.
Short answer: HMRC permanently closed its free online Company Accounts and Tax Online (CATO) filing service on 31 March 2026. All UK limited companies—including micro-entities, property SPVs, and overseas-owned companies—must now file their statutory accounts and Form CT600 using commercial iXBRL-tagged software or through an authorised tax agent. Matthew prepares and files your complete Companies House statutory accounts and HMRC CT600 return online for a transparent fixed fee (from £225 for micro low-volume companies).
Key points
- HMRC shut down its free government CT600 and joint Companies House filing portal on 31 March 2026.
- Every UK Limited Company must submit iXBRL-tagged statutory accounts and a CT600 Corporation Tax computation.
- Companies House accounts are due 9 months after your accounting period ends; Corporation Tax must be paid within 9 months and 1 day.
- Your CT600 return is due 12 months after the end of your accounting period, though it is normally filed simultaneously with your accounts.
- Matthew files complete micro-entity accounts and CT600 returns for £225 fixed fee, or £495 for active trading companies.
Why HMRC closed the free CT600 online portal (CATO)
Until 31 March 2026, directors of small UK limited companies could log into their Government Gateway account and use HMRC's free CATO tool to file basic micro-entity accounts and Form CT600.
Following the closure of that service and the rollout of the Economic Crime and Corporate Transparency Act reforms at Companies House, every UK company must now file digitally using commercial iXBRL software or appoint an authorised agent.
What must be included in an online CT600 submission
HMRC does not accept PDF or spreadsheet attachments for Corporation Tax. A valid online submission requires three synchronised digital components:
- 1. Statutory Accounts (FRS 105 Micro-Entity or FRS 102 1A): Balance sheet, profit and loss notes, and director approval statements
- 2. Full iXBRL Tagging: Machine-readable inline XBRL tags applied to every accounting line item and disclosure note
- 3. Form CT600 & Tax Computation: Adjusting accounting profit for disallowable expenses, capital allowances (AIA / full expensing), marginal relief, and director loan (CT600A) schedules
Fixed-fee CT600 & company accounts filing with Matthew
Instead of buying expensive commercial accounting software and learning iXBRL tagging rules for a single annual filing, you can hand your bank statements or spreadsheet to Matthew.
Get your company accounts & CT600 filed online from £225 fixed fee...
Matthew (Chartered Tax Adviser) prepares your statutory accounts, applies all capital allowances, files your accounts to Companies House, and submits your CT600 electronically to HMRC with zero fuss.
Written and reviewed by Matthew S Manderson CTA ATT AMIT
Chartered Tax Adviser (CTA), Association of Taxation Technicians (ATT), Association of Malta International Taxation (AMIT). General guidance only; tax treatment depends on individual facts.