How to Use HMRC Government Gateway to File Your CT600

Filing your Company Tax Return (CT600) starts with one thing: logging in to HMRC's Government Gateway. Whether you're a first-time director or you've been running your company for years, getting your Gateway credentials set up correctly is essential.

This guide walks you through every step — from creating your account to enrolling for Corporation Tax services and actually submitting your CT600.

What Is the Government Gateway?

The Government Gateway is HMRC's online authentication system. Think of it as the front door to all HMRC digital services. You need a Gateway account to:

File your CT600 Corporation Tax Return
View your company's tax position
Make Corporation Tax payments
Manage your company's tax affairs online

Every limited company that needs to file a CT600 must have a Government Gateway account linked to its Corporation Tax enrolment.

Step 1: Create Your Government Gateway Account

If you don't already have an Organisation Gateway account, here's how to set one up:

Go to gov.uk/log-in-register-hmrc-online-services
Select "Create sign in details"
Choose "Organisation" — this is crucial. Don't select "Individual" as that's for Self Assessment only
Enter your email address — use one you check regularly, as HMRC sends verification codes here
Create a password — must be at least 10 characters with a mix of letters and numbers
Note down your User ID — HMRC generates a 12-digit User ID. Store it somewhere safe. You'll need it every time you log in

Important: Each company should ideally have its own Government Gateway Organisation account. Don't mix personal and company affairs under one login.

Step 2: Enrol for Corporation Tax

Having a Gateway account isn't enough on its own. You must enrol for the Corporation Tax service specifically:

Log in to your Government Gateway account
Go to "Add a tax" or "Get access to a tax, duty or scheme"
Select "Corporation Tax"
Enter your Company UTR (Unique Taxpayer Reference) — this is the 10-digit number HMRC sent when your company registered for Corporation Tax
Complete the security questions — HMRC will ask for details like your company registration number and a recent payment or liability amount
Wait for your activation code — HMRC posts a PIN to your company's registered address. This usually arrives within 7–10 working days

Once you receive the activation code, log back in and enter it to complete enrolment. Only then can you file your CT600 online.

Step 3: Navigating to CT600 Filing

With your account enrolled, here's how to access the filing service:

Log in at gov.uk/log-in-register-hmrc-online-services
Select "Corporation Tax" from your services dashboard
Choose the accounting period you want to file for
Select "File a return" to begin your CT600

HMRC's own online filing tool guides you through each box on the CT600. However, it's worth noting that HMRC's free filing service is being retired — so many companies are switching to commercial software.

Common Login Problems (and Fixes)

"I've forgotten my User ID"

Go to the Gateway login page and select "I've forgotten my User ID." HMRC will email it to the address on file.

"I've forgotten my password"

Select "I've forgotten my password" on the login page. You'll need your User ID and the email address linked to your account.

"My activation code hasn't arrived"

If it's been more than 10 working days, call HMRC's helpline on 0300 200 3410 and request a new code. Activation codes expire after 28 days.

"I'm locked out after too many failed attempts"

After multiple failed login attempts, your account is locked for 2 hours. Wait and try again — or call HMRC if the problem persists.

"I can see the account but Corporation Tax isn't showing"

You likely haven't completed enrolment. Go to "Add a tax" and follow the Corporation Tax enrolment steps above.

Using Third-Party Software Instead

While HMRC's Gateway is the authentication layer, you don't have to use HMRC's own filing interface. Many companies use third-party CT600 software that connects to HMRC via their API. This means:

You still log in via Government Gateway to authorise the software
The software handles the actual CT600 form and calculations
Filing is often faster and less error-prone

Popular options include Taxpipe, which walks you through the CT600 in plain English and files directly to HMRC.

Security Tips for Your Gateway Account

Your Government Gateway account gives access to sensitive company tax data. Protect it:

Enable two-step verification — HMRC supports app-based verification and SMS codes
Don't share your User ID and password — if your accountant needs access, set up an agent authorisation instead
Review access regularly — if a director or employee leaves, update your credentials
Use a strong, unique password — don't reuse passwords from other services

What About Agent Access?

If you use an accountant or tax agent, they can file your CT600 on your behalf using their own Agent Gateway account. You'll need to authorise them by:

Logging in to your company's Gateway account
Going to "Manage your tax agents"
Approving the authorisation request from your accountant

This is the proper way to delegate filing — never share your own login details.

File Your CT600 the Easy Way with Taxpipe

Setting up Government Gateway is just the first step. When it comes to actually completing and filing your CT600, Taxpipe makes it simple. Answer straightforward questions in plain English, and we'll generate your CT600, calculate your Corporation Tax, and file directly to HMRC — no accounting jargon, no expensive fees. Sign up for free and see how easy filing can be.

How to Use HMRC Government Gateway to File Your CT600