Filing a CT600 as a Tax Agent
Filing a CT600 as a Tax Agent — Using Agent Government Gateway Credentials category: ct600-filing
TinyTax supports filing CT600 returns as a tax agent, using your own HMRC Government Gateway credentials. Your client (the company) does not need their own Government Gateway account with Corporation Tax enrolled.
How to File as a Tax Agent
When you reach the Review & Submit page and click to submit to HMRC, a credentials popup appears. It includes a Filing as option:
| Option | Who enters credentials | Used when |
|---|---|---|
| Tax agent | The agent (accountant) | You have HMRC agent authorisation for this client |
| Company director | The company director | The company has their own CT-enrolled Gateway |
What You Need
To file as a tax agent you will need:
- Your agent Government Gateway ID — the 12-digit numeric User ID for your agent Gateway account (not your email address or username)
- Your agent Government Gateway password
- HMRC agent authorisation — your client must have authorised you to act on their behalf through HMRC's agent services. You can check this in your HMRC Online Services account under "Clients"
- The company's Corporation Tax UTR — the 10-digit reference on HMRC's "Notice to deliver a Company Tax Return". This is entered in the main filing form, not the credentials step
Does My Client Need Their Own Government Gateway?
No. When you file as a tax agent, you use your own agent credentials. Your client does not need:
- A Government Gateway account
- Corporation Tax enrolled on their Gateway
- Any HMRC login details
Common Questions
I only have a personal (Individual) Gateway — can I use that?
No. Agent filings require an Organisation Government Gateway account with agent services set up. A personal (Individual) Gateway is for Self Assessment only. If you need to set up agent services, see gov.uk/guidance/get-an-hmrc-agent-services-account.
What if I get Error 1046?
Error 1046 (Authentication Failure) usually means the credentials are incorrect or the account does not have the right authorisation. Common causes:
- Entering your email address or username instead of the 12-digit numeric Gateway ID
- The agent account does not have Corporation Tax services set up
- A temporary HMRC lockout after multiple failed attempts — reset your Gateway password and wait before retrying
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