HMRC CT600 Free Filing Removed: What Every UK Limited Company Must Do Now

From April 1, 2026, HMRC permanently removed its free online CT600 corporation tax filing service. Every UK limited company that previously filed directly through HMRC's portal must now find an alternative — paid third-party software or an accountant. There is no grandfather period. There is no workaround. If your next return is due and you haven't made other arrangements, you cannot file the way you used to.

This change has received almost no mainstream coverage. Most small business owners will discover it when they try to log in and the option is simply gone.

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What Changed and Why It Matters

HMRC's Corporation Tax Online (CATO) service — the free portal that allowed limited companies to file their CT600 returns directly — was discontinued on April 1, 2026.

The government gave notice in 2024, but the communication was buried in technical guidance aimed at software developers and accountants, not at the 5.5 million active limited companies who were quietly using the free service.

What you could do before April 1:

• Log into HMRC's Government Gateway
• Navigate to Corporation Tax
• Complete and file your CT600 return directly, at zero cost

What you can do now:

• Use HMRC-recognised third-party software (paid)
• Use an accountant to file on your behalf (paid)
• File on paper using form CT600 (with the significant risk of penalties for late or incorrect submission)

The free option is gone. HMRC's stated reason: the service was outdated and couldn't support Making Tax Digital's technical requirements. The practical result is that thousands of small companies — particularly those who handled their own tax affairs to keep costs low — now face an unexpected recurring expense.

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Who Is Affected

This affects every UK limited company that files its own corporation tax returns. That includes:

• Small owner-managed businesses filing annual CT600s
• Property investment companies with rental income
• Dormant companies that still file nil returns
• Newly incorporated companies approaching their first accounting period end
• Companies that thought they could defer the change

If you use an accountant who files on your behalf, you may not notice the change — your accountant will have switched to compatible software. But if you file your own returns, the gap in your process is real and immediate.

Approximate scale: HMRC's own estimates suggest around 700,000 companies were using the free CATO service directly. Most of those companies have not yet been contacted about alternative arrangements.

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What Happens if You Try to File Through the Old Method

Nothing. The portal redirects to guidance about third-party software. You cannot submit. If your corporation tax payment deadline arrives and you haven't found an alternative filing method, HMRC will treat the return as unfiled.

Penalties for late CT600 filing:

• 1 day late: £100 flat penalty
• 3 months late: Another £100 flat penalty
• 6 months late: 10% of the estimated unpaid tax
• 12 months late: A further 10% of unpaid tax

These penalties apply even if you owe no tax. A dormant company that misses its filing deadline still faces the flat penalties.

The removal of the free service does not grant an extension. HMRC's position is that alternative options exist and the deadline remains unchanged.

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Your Options Now

Option 1: Use HMRC-Recognised CT600 Software

HMRC maintains a list of software suppliers whose products can file CT600 returns directly via its API. Most are subscription-based, with pricing typically ranging from £100 to £300 per year for small business tiers.

Well-known options include:

• IRIS Elements — used widely by accountants, also available direct to businesses
• Xero Tax — integrated with Xero accounting software
• FreeAgent — included with some NatWest/RBS business banking packages
• TaxCalc — standalone software with a strong CT600 module
• VT Software — popular with small practices and owner-managed businesses

Check HMRC's approved software list at gov.uk/government/publications/software-suppliers-supporting-corporation-tax before purchasing.

Option 2: Engage an Accountant

For companies that file once a year and don't want ongoing software subscriptions, using an accountant or tax agent purely for CT600 filing is often the most cost-effective route.

Typical cost for a straightforward CT600 preparation and filing from an accountant: £200–£600 per year depending on complexity and firm size.

If you take this route, your accountant must be authorised as your agent with HMRC — a simple process through the Government Gateway.

Option 3: Paper Filing (Not Recommended)

You can still file a paper CT600 return by post, but this comes with significant disadvantages:

• HMRC processing times are slower and less reliable
• No confirmation of receipt in real time
• Higher risk of processing errors
• Does not satisfy the Making Tax Digital requirements that will apply to corporation tax in future

Paper filing is a last resort, not a solution.

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How This Connects to Making Tax Digital

The removal of CATO is not an isolated decision. It is part of HMRC's wider push towards Making Tax Digital (MTD) — the programme that will eventually require all taxes to be managed digitally through compatible software.

MTD for Corporation Tax is currently in a voluntary pilot phase, with mandatory implementation expected no earlier than 2026–27 for the largest companies and later for smaller businesses. However, the removal of the free filing service sends a clear signal: HMRC is accelerating the transition away from manual and legacy digital processes.

For companies that adapt now — establishing a software or accountant arrangement — the transition to full MTD for Corporation Tax will be significantly easier. For those who wait, each HMRC deadline brings a new scramble.

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Steps to Take Before Your Next Filing Deadline

Use this checklist to ensure you're not caught out:

1. Check your next corporation tax deadline Your CT600 is due 12 months after the end of your accounting period. Your payment deadline is typically 9 months and 1 day after the period end. Find both dates in your Government Gateway account or on letters from HMRC.

2. Check how you filed last time If you or your bookkeeper filed directly through HMRC's online portal without third-party software, you need to change your process before the next return.

3. Choose your replacement option Software or accountant — decide based on your volume of transactions and your comfort with tax compliance.

4. Register (if using software) Most CT600 software requires you to connect to HMRC's systems via your Government Gateway credentials. Do this before you need to file, not on the deadline date.

5. Don't forget your associated company rules If you have associated companies (e.g., you control two separate limited companies), this affects your corporation tax rates and payment schedules. Make sure your chosen software handles associated company situations correctly.

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What ComplianceAlert Monitors for Limited Companies

ComplianceAlert tracks HMRC procedural changes, Companies House filing deadlines, and tax compliance obligations as they are updated. When HMRC changes how you file, adjusts penalty frameworks, or updates MTD timelines, ComplianceAlert sends you a plain-English alert before the deadline hits.

The removal of free CT600 filing is exactly the kind of change that falls through the cracks — announced in specialist guidance, rarely covered in mainstream business press, and only noticed when it's too late.

Take our free Compliance Score quiz — 20 questions, instant results on where your business stands: compliancealert.co.uk/compliance-score

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Frequently Asked Questions

Can I still use HMRC's free CT600 service after April 1, 2026? No. The service was permanently removed. You must use third-party software or an accountant.

Is there a grace period? No. HMRC gave notice in 2024 and considers sufficient time to have passed for businesses to make alternative arrangements.

Do dormant companies need to change their filing method? Yes. Even if you file a nil return, you must now use approved software or a tax agent.

Does this affect sole traders or partnerships? No. This change only affects limited companies filing CT600 corporation tax returns. Sole traders file Self Assessment, which remains available through HMRC's own service (subject to Making Tax Digital for Income Tax rules from April 2026 onwards for those above the £50,000 threshold).

How do I find an accountant who can file my CT600? ICAEW's find-an-accountant service and the ACCA's member directory both allow you to search for qualified accountants near you. Alternatively, online accountancy services such as Crunch, Tide Accounting, and SJD Accountancy handle CT600 filing remotely.

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Key Takeaways

• HMRC's free CT600 filing service was removed on April 1, 2026. It is gone permanently.
• All UK limited companies must now use third-party HMRC-approved software or an accountant to file corporation tax returns.
• Penalties for late filing start at £100 per day and apply even if no tax is owed.
• This is connected to HMRC's Making Tax Digital programme — adapting now is better than scrambling at the next MTD deadline.
• Start your free 7-day trial at ComplianceAlert — we'll alert you when HMRC and Companies House deadlines and procedures change: compliancealert.co.uk

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