1. Check your HMRC account
Use the official HMRC sign-in link below. Sign in with the Government Gateway user ID and password you intend to use in OneFiler. Check that Corporation Tax is available for the right company.
If you have several Gateway accounts, check which one holds this company’s Corporation Tax access. Access to Self Assessment or VAT alone is not enough. Your OneFiler login and Companies House authentication code are separate.
2. Add Corporation Tax if it is missing
In your business tax account, open Services you can add, find Corporation Tax and choose Enrol for service. Follow HMRC’s prompts.
Have your company registration number, company’s 10-digit UTR, date business started and first accounts end date ready. Use the company’s UTR, not your personal Self Assessment UTR.
3. Activate the service
HMRC posts an activation code to the registered office, normally within 10 days (21 days abroad). Follow the letter’s instructions to activate Corporation Tax using the account where you requested access.
If the letter does not arrive or the code expires, request a replacement through HMRC. Check the registered office address is correct. OneFiler cannot issue or activate the code for you.
4. Return to OneFiler
Once access is active, reopen your filing and enter the same Government Gateway user ID and password, together with the company’s UTR. Check the details carefully before trying the submission again. Your saved figures remain in the filing.
Do not enter the postal activation code or a one-time sign-in code as your Gateway password. Never send your password or activation code to support.
HMRC authentication failure (1046)
This means HMRC refused the sign-in for the submission. Check your Gateway credentials, the company UTR and whether Corporation Tax access is active for that company. This error alone does not tell us which of those checks failed.
Try signing in directly to HMRC. If you cannot sign in, use HMRC’s account recovery help below. If you can sign in but cannot access Corporation Tax, resolve the enrolment or activation first. If HMRC says access has already been requested and you cannot find it, ask HMRC for help before creating another account.
OneFiler says your session has expired
This is a separate sign-in issue. Sign back into OneFiler, reopen the filing and check its submission status before trying again. You do not need to activate Corporation Tax again just because your OneFiler session expired.