If you’re a company director who’s used to filing your annual accounts and Corporation Tax return in one go through HMRC’s website, you’ll need a new plan. The joint filing service, officially known as Company Accounts and Tax Online, or CATO, closed for good on 31 March 2026. If you haven’t switched to a new filing method yet, here’s everything you need to know.
What was the joint filing service?
For years, CATO let small, unrepresented companies do something genuinely convenient: file their annual accounts with Companies House and their Corporation Tax return (CT600) with HMRC in a single online submission. No commercial software, no separate logins, no double data entry. It was aimed squarely at businesses with simple financial affairs — think dormant companies, small landlords who hold property through a company, and some incorporated charities.
That convenience is gone. As of 1 April 2026, the two filings are separate again.
Why did HMRC close it?
HMRC and Companies House have given a few overlapping reasons, but they boil down to this: the service was old, and the rules around it have moved on. Specifically:
- It doesn’t meet modern digital standards. CATO was built years ago and hasn’t kept pace with how filing systems work today.
- Company law has changed. The Economic Crime and Corporate Transparency Act (ECCTA) 2023 introduced new requirements, including identity verification at Companies House, rolling out from autumn 2025, that the old joint service wasn’t built to handle.
- It’s part of a bigger shift. Companies House has announced that filing accounts will only be possible through third-party software as of April 2027. Closing CATO is one step in that direction.
What you need to do
1. Download your filing history now
This is the most time-sensitive step. Once CATO closed, any accounts or returns you filed through it are no longer accessible through that service. HMRC and Companies House have recommended downloading and securely saving at least the last three years of filed accounts and returns. If you haven’t done this yet, do it as soon as possible before the records become harder to retrieve.
2. Choose commercial filing software
From 1 April 2026, you need HMRC-approved commercial software to file your Corporation Tax return, and a separate route to file your accounts with Companies House. If you already use an accountant, this is likely already sorted, most accountants use dedicated software as a matter of course. If you’ve been self-filing, you’ll need to pick a package that supports CT600 submissions and iXBRL-format accounts.
3. Treat the two filings as genuinely separate tasks
This is the part most likely to catch people out. The deadlines haven’t changed:
- Company tax return (CT600): due 12 months after the end of your accounting period
- Annual accounts: due 9 months after your company’s financial year-end (for private limited companies)
What’s changed is that you now need two submissions through two different platforms, rather than one combined filing. It’s worth setting up separate reminders for each deadline, it’s easy to file one and assume the other has gone through automatically when it hasn’t. Missing either one can mean a penalty or a non-compliance notice.
4. Update your internal process
If filing has been a once-a-year task you’ve handled the same way for a while, this is a good moment to write down (or rewrite) your checklist. A quick trial run with your new software, or a conversation with your accountant, before your next filing deadline can save a scramble later.
Who does this actually affect?
If you already use an accountant or commercial software to handle your filings, you’ll likely see little to no disruption, that infrastructure was already in place. The businesses most affected are smaller, unrepresented companies that relied on CATO specifically because it was free and required no extra software: dormant companies, some landlords operating through a company structure, and smaller incorporated charities.
If that’s you, the practical upshot is a small increase in admin (two filings instead of one) and, potentially, a new cost if your chosen software isn’t free. It’s worth shopping around, some providers offer free or low-cost options for companies with simple accounts.
The bottom line
The joint filing service closed earlier this year, and from 1 April 2026 onward, filing your company accounts and Corporation Tax return is a two-step process using separate platforms. If you haven’t already downloaded your filing history or lined up new software, that’s the priority now. After that, it’s mostly a matter of building the extra step into your annual routine so nothing slips through the cracks.