CATO closed on 31 March 2026. Your obligations have not changed, but the way you meet them has.
The short version
- CATO closed on 31 March 2026. HMRC’s free filing service has not been replaced.
- Corporation Tax returns now go through commercial software. Paper is accepted only with a reasonable excuse, or in Welsh.
- Companies House was not affected. WebFiling is still open and still free until 31 March 2028.
- Your deadlines have not moved. CT600 at 12 months, accounts at 9 months, payment at 9 months and 1 day.
- Late filing costs more than it used to. The opening HMRC penalty doubled to £200 in April 2026.
HMRC's Free CT600 Filing Service Has Closed
For years, a small company could file its own Corporation Tax return without paying anyone. HMRC’s Company Accounts and Tax Online service, usually shortened to CATO, let you file your CT600 with HMRC, your annual accounts with Companies House, or both at once, from one set of figures and at no cost. DataTracks estimates that around 350,000 UK companies were filing through it.
It closed on 31 March 2026 and has not been replaced.
If you used it, nothing about what you owe or when you owe it has changed. What has changed is the route. This is what closed, what did not, and what to do before your next deadline.
What CATO was, and who could use it
CATO was built for companies filing for themselves. HMRC designed it around a company’s own filing credentials and never opened it to agents, so accountants could not use it on a client’s behalf.
It was never open to every company. Eligibility was limited to micro-entities and small companies that were exempt from audit and had straightforward tax affairs, with turnover up to £632,000 a year. A company could not use it if any of the following applied:
- its accounts required an audit, or had been audited
- it was part of a group
- it was not registered in the UK
- it was in liquidation or receivership
- it paid its Corporation Tax in instalments
- its accounting period was covered by more than one set of statutory accounts
- it needed to reclaim a loan to a participator, such as a director’s loan, more than nine months after the end of the accounting period
- it was an insurance company (independent brokers excepted), an investment company, a credit union or a commercial property management company
Charities had a separate ceiling of £6.5 million turnover.
What changed on 1 April 2026
Corporation Tax returns must now be filed using commercial software. HMRC publishes a list of recognised suppliers, and any of them can file a CT600 on your behalf or give you the software to do it yourself.
A paper return is accepted only in two situations: if you have a reasonable excuse for not being able to file online, or if you are filing in Welsh. Not having software is not a reasonable excuse, and neither is CATO having closed.
Your deadlines have not moved.
| What | Filed with | Deadline |
|---|---|---|
| Company Tax Return (CT600) | HMRC | 12 months after the end of your accounting period |
| Corporation Tax payment | HMRC | 9 months and 1 day after the end of your accounting period |
| Annual accounts | Companies House | 9 months after the end of your financial year |
| First annual accounts | Companies House | 21 months after the date you registered |
Note the order. Your Corporation Tax falls due roughly three months before the return that works out how much it is. Most companies prepare the return early in order to know what to pay.
What has not changed, for now
Companies House filing is separate and was not affected by CATO’s closure. This catches people out, because CATO handled both and its closure is widely read as having ended both.
You can still file your annual accounts with Companies House through its own WebFiling service, free of charge, or on paper, or through commercial software. That remains true until 31 March 2028.
What changes at Companies House in 2028
Under the Economic Crime and Corporate Transparency Act, Companies House is overhauling how accounts are filed. WebFiling closes for accounts on 31 March 2028, and from 1 April 2028:
- Paper filing closes for accounts too. WebFiling stays open for everything else, such as confirmation statements and changes of director.
- Every UK registered company must file its accounts in iXBRL format using commercial software, whether it files itself or through an agent.
- Micro-entities and small companies must file a profit and loss account. They will be able to opt out of having it published on the public register, and Companies House has said it will confirm how that works nearer the time.
- The option to file abridged accounts is removed.
- Companies claiming audit exemption must provide a strengthened eligibility statement.
This date has already moved once, from April 2027, so treat it as firm but not immovable. The practical point for a small company is that the balance-sheet-only filing most micro-entities use today will not survive it.
What it costs to miss a deadline
HMRC doubled its fixed Corporation Tax late filing penalties for returns with a filing date on or after 1 April 2026.
| How late | Before 1 April 2026 | From 1 April 2026 |
|---|---|---|
| 1 day | £100 | £200 |
| More than 3 months | £200 in total | £400 in total |
| 6 months | 10% of unpaid tax | 10% of unpaid tax |
| 12 months | A further 10% | A further 10% |
File late three years running and the fixed penalties rise again, to £1,000 on day one and £2,000 after three months.
Companies House runs its own separate penalty regime for late accounts. For a private company it is £150 up to a month late, £375 between one and three months, £750 between three and six, and £1,500 beyond six months. File late in two successive financial years and the penalty doubles.
Both sets can apply to the same company in the same year, because they are two different filings to two different bodies.
What to do now
- Find your two deadlines. They are different dates and they are set by your accounting period, not by the tax year. Both appear on your Companies House record.
- Check your company size. The micro-entity and small company thresholds rose in April 2025, so the category you were in may not be the one you are in now.
- Choose software before you need it. Leaving it until the week of the deadline is how penalties happen, and some providers need you to set up a Government Gateway link before you can file.
- Decide how you will file with Companies House. WebFiling still works. If you would rather do both filings in one place, software that handles both saves re-entering the same figures.
- Retrieve anything you left in CATO. Filings you did not download before it closed have to be requested from HMRC or Companies House directly.
- File anything overdue straight away. The penalties escalate with time, and CATO’s closure is not a reasonable excuse.
What to look for in filing software
Not every product does the whole job. Before you buy, check it covers:
- The CT600 itself, and that the supplier appears on HMRC’s recognised list.
- Corporation Tax computations, not just the form. HMRC requires the computation alongside the return.
- iXBRL tagged accounts. HMRC will not accept accounts in any other format, and from April 2028 neither will Companies House. If the software does not produce iXBRL, you will be paying someone else to convert the file.
- Both filings, if you want them from one set of figures rather than keying the same numbers twice.
- Validation before submission, so errors surface while you can still fix them rather than as a rejection.
- A filing receipt. HMRC returns an IRmark on acceptance. Keep it.
Frequently Asked Questions
Is there still a free way to file a CT600?
No. HMRC withdrew the only free route when CATO closed on 31 March 2026. Every Corporation Tax return now goes through commercial software, unless you qualify to file on paper.
Do I still have to file with Companies House as well?
Yes, if your company is required to file annual accounts, which almost all are. It is a separate filing with a separate deadline, and CATO’s closure did not change it.
Can I still use Companies House WebFiling?
Yes, and it is still free. It closes for accounts on 31 March 2028, after which accounts must go through commercial software in iXBRL format.
What is iXBRL, and why do I need it?
iXBRL is the format HMRC requires for digital accounts and tax computations. It is a version of your accounts that a computer can read as well as a person, with each figure tagged so it can be identified automatically. Companies House will require it too from April 2028.
Was CATO's closure a reasonable excuse for filing late?
No. HMRC gave notice well in advance and penalties apply as normal.
I can't find my old CATO filings. What do I do?
Request them from HMRC for Corporation Tax returns, and from Companies House for accounts. Anything not downloaded before the service closed is no longer accessible through it.
If you are not sure whether your company counts as a micro-entity under the current rules, the thresholds changed in April 2025 and our guide to company size thresholds sets out what applies now.
DataTracks Nova is our own filing software for micro-entities, covering the CT600, iXBRL accounts and Companies House from one set of figures. Former CATO users get their first CT600 free.
About DataTracks
DataTracks is a software and services provider for regulatory reporting, working with businesses in more than 25 countries since 2005.
DTracks Limited is recognised by HMRC as an iXBRL Managed Tagging Service provider and as a commercial software supplier supporting Pillar 2 returns, and by Companies House as an authorised software provider for filing company accounts.
