Thanks for getting in touch. The "Not authorised to use this service" error is a common HMRC Government Gateway issue with a few possible causes. Let me walk you through them:
1. Check you're using an Organisation Gateway account
Corporation Tax requires an Organisation (business) Government Gateway account, not a personal one. If you're using your personal Gateway (e.g., the one for Self Assessment), you'll need to create a separate Organisation account:
- Go to https://www.gov.uk/log-in-register-hmrc-online-services
- Select "Organisation" when prompted
- You'll need your company's 10-digit Corporation Tax UTR
2. Check another Gateway isn't already linked
If a previous accountant or agent set up Corporation Tax access for your company, their Gateway account may already be linked. In this case, HMRC needs to unlink it before you can register.
3. Check you're using the correct UTR
Make sure you're entering your company's Corporation Tax UTR (10 digits), not any personal UTR you might have. Your company's CT UTR would have been sent by HMRC shortly after incorporation.
What to do next:
If you've checked the above and still can't add Corporation Tax, I'd recommend calling HMRC's Online Services Helpline on 0300 200 3600. Have your company's UTR ready, and they can:
- Confirm whether a Gateway is already linked to your company
- Unlink any old accounts if needed
- Help you set up fresh access
Once Corporation Tax is activated on your Gateway, you'll be able to submit your CT600 through TinyTax.
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1. Check you're using an Organisation Gateway account
Corporation Tax requires an Organisation (business) Government Gateway account, not a personal one. If you're using your personal Gateway (e.g., the one for Self Assessment), you'll need to create a separate Organisation account:
- Go to https://www.gov.uk/log-in-register-hmrc-online-services
- Select "Organisation" when prompted
- You'll need your company's 10-digit Corporation Tax UTR
2. Check another Gateway isn't already linked
If a previous accountant or agent set up Corporation Tax access for your company, their Gateway account may already be linked. In this case, HMRC needs to unlink it before you can register.
3. Check you're using the correct UTR
Make sure you're entering your company's Corporation Tax UTR (10 digits), not any personal UTR you might have. Your company's CT UTR would have been sent by HMRC shortly after incorporation.
What to do next:
If you've checked the above and still can't add Corporation Tax, I'd recommend calling HMRC's Online Services Helpline on 0300 200 3600. Have your company's UTR ready, and they can:
- Confirm whether a Gateway is already linked to your company
- Unlink any old accounts if needed
- Help you set up fresh access
Once Corporation Tax is activated on your Gateway, you'll be able to submit your CT600 through TinyTax.
Let me know if you have any other questions!