Filing CT600 as a Tax Agent

TinyTax supports filing CT600 returns as a tax agent — an accountant or bookkeeper submitting on behalf of a client company. This guide explains how to use agent credentials and what to do if you receive an error.

How Agent Filing Works

When you reach the submission step, you'll see a "Filing as" option:

Option	Who it's for	Credentials to use
Company director	Directors filing their own return	The company's own Government Gateway
Tax agent	Accountants filing for a client	Your own agent Government Gateway

Select Tax agent, then enter:

• Company UTR — the client company's 10-digit Unique Taxpayer Reference
• Government Gateway User ID — your own 12-digit agent Gateway ID
• Password — your agent Gateway password

Your client doesn't need to be present or provide their own Gateway credentials.

Before You Start: HMRC Requirements

For agent credential submission to succeed, two things must be in place:

1. You need an agent Government Gateway account

Your agent Gateway account must have Corporation Tax enrolled. Log into HMRC Online Services for Agents and check that Corporation Tax appears under your services. If it doesn't, you'll need to enrol.

2. You must be authorised for this client

HMRC must have an active agent authorisation on file for the client company. Without it, HMRC will reject the submission even if your credentials are correct. Authorisation can be set up via:

• 64-8 form (traditional paper authorisation) — once processed by HMRC, appears in your agent account
• HMRC Online Services for Agents — go to "Manage authorisations" to check or request authorisation

If authorisation is pending or has expired, the submission will fail at the HMRC authentication step.

Common Agent Credential Errors

"HMRC did not accept your Government Gateway credentials"

This is the most common error. Work through these checks in order:

1. Confirm you're using your agent Gateway ID, not your personal one

Your agent business Gateway ID is the 12-digit ID you use to log into HMRC Online Services for Agents. It's different from any personal Government Gateway account.

2. Check the company UTR is correct

The UTR must be the client company's 10-digit Corporation Tax reference. Find it on HMRC's "Notice to deliver a Company Tax Return" letter, or in your HMRC agent account under the client's record.

3. Verify your agent account has CT enrolled

Log into your agent Gateway directly at gov.uk to confirm you can access Corporation Tax services. If CT isn't enrolled, you won't be able to file via any software.

4. Check authorisation is active

In HMRC Online Services for Agents, check "Manage authorisations" for the client company. If it shows as expired or missing, you'll need to re-authorise before submitting.

5. Reset your password if needed

If you've recently changed your password or had login issues, try logging into gov.uk directly first to confirm your credentials work.

Common Questions

My credentials work for other clients — why is this one failing?

The most likely cause is missing or expired client-specific authorisation. Your credentials might be correct, but HMRC needs to see an active authorisation for this particular company.

Can I use the client's Gateway credentials instead?

Yes — if you switch "Filing as" to Company director and use the client's own Gateway credentials, that will also work if the client has their company's CT enrolled. However, you'll need to obtain those credentials from the client.

I've checked everything and it still fails — what's the error message?

The exact error message from HMRC will help diagnose the problem. Contact hello@tinytax.co.uk with the full error text and the company number.