Enrol for Corporation Tax online before you file a CT600

By Max Szolcek, founder of Dormant Return. Last updated 17 September 2026

Every piece of filing software, including this one, submits a Company Tax Return under the company's own Government Gateway sign-in. That sign-in only works for Corporation Tax once the Corporation Tax service has been added to the account and activated with a code HMRC sends by post. Directors who skip this step meet a sign-in failure the moment they try to file. Here is what the step involves and how long it takes.

Registered is not the same as enrolled

Two things happen with similar names, and it helps to keep them apart.

Registering the company for Corporation Tax is between HMRC and the company. When you register a company at Companies House you are usually set up for Corporation Tax at the same time, unless the company is dormant, and HMRC posts the company's 10-digit Unique Taxpayer Reference (UTR) to the registered office. gov.uk says that if the UTR has not arrived within 15 working days of registration you can request it online. This is the number in how to find your company's Corporation Tax UTR.
Enrolling for Corporation Tax online is between HMRC and a Government Gateway account. It links the account to the company's UTR so the account can see the company's Corporation Tax record and file its returns. This is the part that filing software depends on.

A company can be registered for Corporation Tax, with a UTR and a notice to deliver a return, and still have no Gateway account enrolled to file for it. That is the usual position for a small company that has always used an accountant, or that was incorporated by a formation agent and then left alone.

What you need before you start

gov.uk lists four things for adding Corporation Tax services to a business tax account:

the company's registration number;
the date the company started to do business;
the date the company's first accounts are made up to;
the company's 10-digit Corporation Tax UTR.

You also need a Government Gateway user ID and password for the company. HMRC's sign-in page offers two kinds of sign-in: a Government Gateway user ID of up to 12 characters with a password, or a GOV.UK One Login with an email address and password. Software filing uses the Government Gateway user ID and password, so that is the one to have ready. If the company has never used HMRC online services, choose "Create new sign in details" on the sign-in page and set the account up for the organisation rather than for you personally.

Adding Corporation Tax to the account

The steps on gov.uk are short:

Sign in to the business tax account with the Government Gateway user ID and password.
Choose "Services you can add" from the menu.
Find Corporation Tax and choose "Enrol for service", then enter the details above.

The enrolment is then pending rather than live. HMRC posts an activation code to the company's registered office address, within 10 days according to gov.uk (21 days if the address is abroad), with instructions for activating the service. Until you sign in and enter that code, the account cannot file a Corporation Tax return. The same letter states the company's Corporation Tax payment deadline; for a dormant company with no tax to pay, that date is not one to worry about.

Once the enrolment is active, Dormant Return files a dormant company's CT600 with iXBRL accounts for £5 under that sign-in, in about five minutes, with an automatic refund if HMRC rejects the return. The password is used for the submission and never stored.

Start your return

What happens if you file before enrolling

The Government Gateway refuses the sign-in with error 1046, "Authentication Failure. The supplied user credentials failed validation for the requested service." It is the same message for a wrong password, a One Login sign-in used where a Gateway user ID was needed, and an account with no active Corporation Tax enrolment, which makes it a confusing error to receive. The return is never looked at, so nothing is filed and nothing is at risk. With Dormant Return the return stays paid and prepared, and you file again once the enrolment is active. Normally a paid return is kept for 24 hours, but while you wait for the activation code you can ask, on the filing page, for it to be kept for 30 days from payment, so you can file as soon as the code arrives without paying again. If it still has not been filed by then, the fee is refunded automatically. If the problem is the sign-in itself rather than the enrolment, see find or recover your Government Gateway user ID and password. The full list of what each HMRC error means is in CT600 rejected by HMRC: what the error codes mean.

Timing against the filing deadline

Postal activation is the slow part. If the return is due soon, enrol first and prepare the return while the code is in the post: a Company Tax Return is due 12 months after the end of the accounting period it covers, and the flat late-filing penalties start the day after that date whether or not any tax is owed. The ladder is in late CT600 penalties for dormant companies. A company that expects to stay dormant should also consider telling HMRC it is dormant, which can stop future notices to deliver a return being issued at all.

If an accountant used to file for you

An accountant files through their own agent account, authorised against the company's UTR. That authorisation does not give the company a Gateway sign-in of its own, so when the accountant stops acting, the company usually has to enrol from scratch as above. The accountant should be able to confirm the UTR and the accounting period HMRC expects, both of which you will need.