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

HMRC's free Company Accounts and Tax Online (CATO) service closes on 31 March 2026. This guide walks you through switching to commercial software — 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 — Free for micro-entity companies
  • 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: Download Your HMRC Filing History

Do this first. After 31 March 2026, you won't be able to access your previous returns filed through CATO.

How to Download

  1. Log in to your HMRC business tax account at tax.service.gov.uk
  2. Navigate to Corporation Tax
  3. Find "View submitted returns" or similar
  4. Download each return from the last 3+ years
  5. Save the PDFs securely (backup to cloud storage)

What to Download

  • All CT600 returns
  • Corporation tax computations
  • Submitted accounts
  • Any correspondence or acknowledgements
Keep these records for at least 6 years — HMRC can enquire into returns up to 6 years old (longer if fraud suspected).

Step 2: Create Your Account

On TinyTax

  1. Visit tinytax.co.uk/register
  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: Set Up HMRC Connection

To submit CT600 returns, your software needs to connect to HMRC's systems.

Authorise Your Software

  1. In TinyTax, go to Settings → HMRC Connection
  2. Click "Connect to HMRC"
  3. You'll be redirected to Government Gateway
  4. Log in with your existing credentials
  5. Grant permission for TinyTax to file on your behalf
Note: You're authorising the software to submit returns. Your Government Gateway account remains unchanged.

Enter Your UTR

Your Unique Taxpayer Reference connects your company to HMRC's records:

  1. Enter your 10-digit UTR
  2. TinyTax validates it against HMRC's systems
  3. You're ready to file

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

If you've previously filed through HMRC, TinyTax can fetch your prior year figures automatically to ensure consistency.

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 "Import Trial Balance"
  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: From April 2027, Companies House will also require software filing. TinyTax handles both.

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
NowDownload HMRC filing history
Before your next filingCreate account and set up software
Next filing dueFile through new software (practice run)
31 March 2026HMRC service closes permanently
Don't wait until the deadline. Switch with your next return to learn the software without pressure.

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 free support for all users.

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

Not after 31 March 2026 — the service closes permanently. But you can switch between commercial software providers anytime.

Is my data safe when switching?

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

Summary

Switching from HMRC's CT600 service is straightforward:

  1. Download your HMRC filing history (do this now)
  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. Create your free account →