Switch from HMRC's CT600 Service | TinyTax Support

Switch from HMRC's CT600 Service

Step-by-step guide to switching from HMRC's free CT600 filing service, which closed on 31 March 2026, to commercial software.

How to Switch from HMRC's CT600 Service: Step-by-Step Guide

HMRC's free Company Accounts and Tax Online (CATO) service closed permanently on 31 March 2026. Corporation Tax returns now have to be filed through commercial software. This guide walks you through switching — it's simpler than you might think.

Before You Start

What You'll Need

  • Your company's Government Gateway credentials
  • Your Unique Taxpayer Reference (UTR) — 10-digit number from HMRC
  • Your company registration number — 8-digit number from Companies House
  • Your latest accounts or trial balance
  • About 30 minutes for the initial setup

Choose Your New Software

If you haven't already, select your CT600 filing software. Options include:

  • TinyTax — £20/year + VAT (dormant), £59/year + VAT (one trading company)
  • TaxCalc — From £100+/year
  • IRIS — From £200+/year
  • Sage — From £150+/year
This guide uses TinyTax as an example, but the process is similar for other software.

Step 1: Find Your Old HMRC Filing History

Returns you filed through CATO are no longer available to download — that access went when the service closed on 31 March 2026.

If you saved copies before it closed, keep them. Hold your records for at least 6 years — HMRC can enquire into returns up to 6 years old, longer if fraud is suspected. Useful things to have to hand:

  • Previous CT600 returns
  • Corporation Tax computations
  • Previously submitted accounts
  • Any correspondence or acknowledgements
If you have nothing saved, you can still file going forward. You will need last year's closing figures as your comparatives — your accountant, your bookkeeping records, or the accounts filed at Companies House will have them. TinyTax can also fetch your prior year figures from a previous filing where one exists.

Step 2: Create Your Account

On TinyTax

  1. Visit TinyTax
  2. Enter your email address
  3. Create a password
  4. Verify your email

Link Your Company

  1. Enter your company registration number
  2. TinyTax automatically fetches your company details from Companies House
  3. Confirm the details are correct

Step 3: How TinyTax Submits to HMRC

There's no upfront "connect" or "authorise" step to set up. CT600 filing in TinyTax works differently to MTD VAT — there's no OAuth redirect, no Settings page where you grant standing access, and no "Connect to HMRC" button.

Instead, when you submit your return:

  1. TinyTax opens the submission modal
  2. You enter your Gateway User ID and password
  3. You enter your company UTR (10 digits)
  4. TinyTax bundles your CT600 and sends it directly to HMRC's Transaction Engine, including those credentials in the submission
This is HMRC's design for Corporation Tax — the API requires credentials per submission. Every commercial CT600 product works the same way. Your password isn't stored unless you tick the "remember" option in the submission modal.

If you've previously connected to HMRC for MTD VAT, you'll have seen the OAuth-style "Connect to HMRC" button and a redirect to Government Gateway. That's an MTD-only mechanism — CT600 doesn't use it.

Step 4: Enter Your Accounting Period

Set up the period you're filing for:

  1. Select your accounting period end date
  2. Confirm the period start date
  3. If it's your first year, enter the incorporation date

For Ongoing Filings

TinyTax fills in your prior year balance sheet comparatives automatically from your last accounts on the Companies House register. Note that this comes from the public register, not from HMRC — a CT600 you filed with HMRC cannot be retrieved by any software, so your profit and loss figures are entered fresh for the current year.

Step 5: Import or Enter Your Figures

Option A: Trial Balance Import (Recommended)

If you use accounting software like Xero or QuickBooks:

  1. Export your trial balance as CSV
  2. In TinyTax, click the Import button next to the Profit & Loss or Balance Sheet heading
  3. Upload the CSV file
  4. Map accounts to CT600 categories (TinyTax suggests mappings)
  5. Review and confirm

Option B: Manual Entry

Enter figures directly into the submission form:

  1. Turnover and other income
  2. Cost of sales and expenses
  3. Assets and liabilities
  4. Any adjustments for tax purposes

Step 6: Review and Calculate Tax

TinyTax automatically:

  • Calculates your Corporation Tax liability
  • Applies the correct tax rates (19% small profits, 25% main rate)
  • Computes marginal relief if applicable
  • Generates your CT600 return
  • Creates iXBRL-tagged accounts

Check for Errors

The software validates your return before submission:

  • Mathematical accuracy
  • Required fields completed
  • Logical consistency
  • HMRC rule compliance
Fix any errors highlighted before proceeding.

Step 7: Submit to HMRC

When you're ready:

  1. Review the complete CT600 return
  2. Click "Submit to HMRC"
  3. The software transmits your return electronically
  4. HMRC sends an acknowledgement (usually within minutes)
  5. Save the submission receipt

What Gets Submitted

  • CT600 Corporation Tax return
  • Corporation Tax computation
  • iXBRL accounts (attached to the CT600)

Step 8: File Accounts with Companies House

You can file your accounts with Companies House through TinyTax simultaneously or separately:

  1. Review the accounts generated
  2. Click "Submit to Companies House"
  3. Receive confirmation and submission number
Important: Companies House WebFiling for accounts also closed on 31 March 2026, so accounts already have to be filed as iXBRL through software. TinyTax handles both filings.

Step 9: Pay Your Corporation Tax

Submitting your return doesn't pay your tax. You still need to pay separately:

Payment Methods

  • Direct Debit — Set up through HMRC
  • Bank transfer — Using HMRC payment details and your UTR
  • Corporate credit/debit card — Via HMRC's website

Payment Deadline

Corporation Tax is due 9 months and 1 day after your accounting period ends.

Timeline: When to Switch

TimingAction
31 March 2026HMRC's free CT600 service closed permanently
NowGather last year's figures and your Gateway credentials
Before your next filingCreate an account and add your company
Next filing dueFile through the new software
Don't leave it to the filing deadline. Setting up well before the return is due means any Government Gateway problems surface while you still have time to sort them out.

Frequently Asked Questions

Do I need to tell HMRC I'm switching?

No. HMRC doesn't require notification when you change filing software.

Will HMRC know my return came from different software?

Yes, returns include the software name, but this doesn't affect how HMRC processes them.

What if I have a problem during my first submission?

Contact your software provider's support. TinyTax offers affordable support for all users.

Can I go back to HMRC if I don't like the new software?

No — the HMRC service closed permanently on 31 March 2026. But you can switch between commercial software providers at any time.

Is my data safe when switching?

Reputable software providers use encryption and secure connections. TinyTax uses bank-grade security.

Can new companies still register for Corporation Tax after 31 March 2026?

Yes. The HMRC service that's closing is their free CT600 filing tool — not the Corporation Tax registration system. New companies can still register for Corporation Tax and receive a UTR after 31 March, exactly as before. The Government Gateway login system also remains open.

I'm not a registered agent — I just help clients file. What can I use?

You don't need to be a registered HMRC agent to use commercial software. When submitting a CT600 through TinyTax, choose Company director at the submission step and enter your client's own Government Gateway credentials (their Gateway ID and password). You handle the filing; they provide their login. No agent registration is needed for this approach.

My client has a UTR but no Government Gateway account. What should they do?

They need to create an Organisation Government Gateway account and enrol for Corporation Tax — this process is unaffected by the 31 March closure. Steps:

  1. Go to gov.uk/log-in-register-hmrc-online-services and create an Organisation account
  2. Select Add a tax, duty or scheme → Corporation Tax
  3. Enter their UTR and company registration number to enrol
  4. Once enrolled, they (or you, via Company director mode) can file through commercial software
HMRC may send an activation code by post — if so, they'll need to wait for it before the enrolment is complete.

Summary

Switching from HMRC's CT600 service is straightforward:

  1. Gather last year's figures and your Government Gateway credentials
  2. Create an account with your chosen software
  3. Set up HMRC connection via Government Gateway
  4. Import or enter your figures
  5. Submit your return through the new software

Ready to switch? TinyTax makes the transition simple. Get started →

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