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 |
- 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
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 does not, you will 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
Companies House Accounts: Auth Code Still Required
Filing accounts with Companies House uses a separate authentication mechanism from the HMRC Government Gateway. Whether you are filing as an agent or a company director, the client's Companies House authentication code is always required.
| Step | Credential used |
|---|---|
| Submit CT600 to HMRC | Your agent Gateway User ID + password |
| Submit accounts to Companies House | Client's 6-character Companies House auth code |
If the client does not have their auth code: They can request a new one from Companies House at find-and-update.company-information.service.gov.uk — log in and go to the company's profile, then "Request authentication code". It arrives by post to the registered office within 5 working days.
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 are 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 is 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 is not enrolled, you will not 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 will need to re-authorise before submitting.
5. Reset your password if needed
If you have 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 will need to obtain those credentials from the client.
Do I need the client's Companies House auth code even though I am an authorised agent?
Yes. The Companies House auth code is always required regardless of your agent status. It is specific to the company's registered address and is not linked to HMRC agent authorisation.
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 [email protected] with the full error text and the company number.
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