HMRC's Free CT600 Filing Service Has Closed
Introduction
For over a decade, CATO was the quiet workhorse behind hundreds of thousands of UK company filings. If you ran a small owner-managed business or a buy-to-let company, chances are you filed your Corporation Tax return through it without ever needing an accountant or a subscription. That option is gone. As of 1 April 2026, every company that used to rely on it needs commercial software – and if you haven’t sorted that yet, this is the moment to fix it, not the moment to wait and see what happens.
What Was CATO, and What Exactly Closed
Company Accounts and Tax Online – officially titled on GOV.UK as “File your accounts and Company Tax Return” – had been running since 2011 as a joint service from HMRC and Companies House. It let eligible companies do one of three things, all for free:
- Submit a CT600 Corporation Tax return to HMRC
- Submit annual accounts to Companies House
- Submit both together in a single online session
It was designed specifically for micro-entities and small companies with straightforward affairs and no need for a statutory audit.
HMRC’s stated reasons for closing it were that the service no longer met modern digital standards and didn’t align with the direction of company law reform under the Economic Crime and Corporate Transparency Act, alongside ongoing public sector funding constraints. The government gave over a year’s notice – the closure was first flagged in guidance published March 2025 – but take-up of alternatives lagged behind that notice period for a meaningful share of users.
The New Rules From 1 April 2026
Corporation Tax (CT600)
you must file using commercial software. Paper filing is only available if you have a reasonable excuse or are filing your return in Welsh - for everyone else, online via commercial software is the only route.
Companies House accounts
this is genuinely a separate matter. You still need to file annual accounts with Companies House every year, and that hasn't changed because of CATO's closure - for now, you can continue using Companies House's own online filing route (Web Filing), third-party software, or post. That's changing too, though: Companies House has confirmed that from 1 April 2028, web and paper filing for accounts will close permanently, and all companies will need to file using commercial software in iXBRL format.
Who's Actually Affected
The honest answer: almost every micro and small company that filed for itself.
CATO itself was aimed at this broad micro and small population - one-person consultancies, family businesses, buy-to-let vehicles, and dormant holding companies whose directors have never used dedicated accounting software.
According to data DataTracks received directly from HMRC, more than 350,000 companies used CATO to file their returns during the 2025-2026.
If You Haven't Switched Software Yet
If your company's filing deadline has already passed since 31 March 2026 and you haven't filed, the CATO closure doesn't pause the clock - HMRC's standard late filing penalties for CT600 still apply regardless of why the return is late, and they escalate the longer a return stays outstanding. The priority now is simply to get compliant software in place and file, not to wait for a further extension that isn't coming.
If you filed through CATO in previous years and didn't download your historic returns and accounts before the cutoff, you can't retrieve them through the old portal anymore - you'd need to request copies directly from HMRC (Corporation Tax office) or Companies House instead.
Checklist: What to Do Now
1
Confirm your company's next CT600 and accounts filing deadlines
2
Check whether the April 2025 size threshold changes affect your company's classification
3
Choose a commercial software that covers CT600, computations, and automated iXBRL generation
4
Separately confirm how you'll file Companies House accounts, since that's no longer bundled with your CT600
5
If you have historic CATO filings you didn't download, contact HMRC or Companies House directly for copies
6
If a deadline has already passed, file as soon as possible to limit escalating late-filing penalties
7
Check whether a dormant company filing exemption applies to you, if relevant
Choosing Commercial Software: What to Check For
Not all “commercial software” on HMRC’s recognised list does the same job. Before choosing, confirm the product covers what you need:
CT600 filing
the Corporation Tax return itself
Computations
the Corporation Tax computation that sits behind the return
iXBRL-tagged accounts
HMRC requires accounts and computations to be submitted in iXBRL format, not as plain PDFs
Combined filing capability
check whether the software can submit to both HMRC and Companies House together, or whether you'll need to arrange your Companies House filing separately
Frequently Asked Questions
Can I still file my CT600 for free?
Not through a government portal. HMRC no longer offers a free online CT600 filing route for the general company population – paper filing remains, but only if you have a reasonable excuse or are filing in Welsh. Some commercial software providers do offer free tiers for very simple returns, so “free” isn’t entirely off the table, but it’s no longer a government-run option.
Does this affect my Companies House accounts filing too?
Not directly, for now. Companies House filing continues through its existing channels (Web Filing, software, or post) — what’s closed is specifically the joint HMRC service that let you file both at once. That said, Companies House has confirmed that Web Filing and paper filing for accounts will close from 1 April 2028, after which commercial software will be mandatory there too.
I missed my filing deadline because I didn't know CATO had closed - what happens?
Standard late filing penalties still apply. HMRC’s guidance doesn’t treat the closure itself as a reasonable excuse, so the priority is to get software in place and file as quickly as possible.
Can I get my old CATO-filed accounts and returns back?
Not through the old portal – it’s been switched off. You’d need to contact HMRC’s Corporation Tax office or Companies House directly to request copies of historic filings.
Is this the same as the Companies House software-only filing change?
No – they’re related but separate. This closure is specifically about HMRC’s joint CATO service ending. Companies House has its own, later timeline for moving accounts filing to software-only, which is a different reform under ECCTA.
