Corporation Tax activation code: get set up to file yourself

In short: To file your own Company Tax Return (CT600), the company needs its own Government Gateway login with Corporation Tax added. You add it with the company's UTR, and HMRC posts an activation code to the registered office within 10 days, or 21 if you live abroad (GOV.UK: Add Corporation Tax to your business tax account). Use it within 28 days of the letter's date (HMRC: sign-in and registration FAQs).

Before you start, take the 30-second check to see whether Taxley fits your company, or get the company's filing dates from the Corporation Tax deadline calculator.

Which codes and logins do you need to file company tax yourself?

You need four things from two bodies. From HMRC: the company's 10-digit UTR, a Government Gateway user ID, and an activation code that switches Corporation Tax on (GOV.UK: Add Corporation Tax to your business tax account). From Companies House: the company's 6-character authentication code for filing accounts (GOV.UK: Company authentication codes for online filing). A director's personal code proves who you are, not the company's right to file.

Which code or login is which (checked on GOV.UK on 26 September 2026)

Sources: adding Corporation Tax, HMRC sign-in help, authentication codes, personal codes and identity checks.

The HMRC activation code and the Companies House authentication code do different jobs and never stand in for each other. The activation code is a one-off: HMRC sends one code per service, and you use it to switch that service on in the company's online account (HMRC: sign-in and registration FAQs). The authentication code stays the same until the company changes it, and Companies House treats it as the equivalent of a company officer's signature on everything filed online. Companies House also says a Government Gateway account can't be used to send it information (GOV.UK: Company authentication codes for online filing); its WebFiling service has needed a GOV.UK One Login to sign in since 13 October 2025 (GOV.UK: Filing your Companies House information online). The personal code belongs to a person rather than the company.

How do you add Corporation Tax to a Government Gateway account, step by step?

Sign in to or create the company's business tax account, choose "Services you can add", enrol for Corporation Tax with the company's UTR, then wait for HMRC's activation code by post and enter it (GOV.UK: Add Corporation Tax to your business tax account). Allow about 45 minutes of hands-on time; the postal waits come on top.

Step 1: Find the company's 10-digit UTR

Find the company's Unique Taxpayer Reference first, because the Corporation Tax enrolment asks for it (GOV.UK: Add Corporation Tax to your business tax account). HMRC posts it to the company after incorporation. If it hasn't arrived 15 working days after the company was registered, request a copy online; HMRC posts it to the address held by Companies House (GOV.UK: Find your UTR number). The guide to finding a UTR lists every place to look.

Step 2: Sign in to or create the company's Government Gateway user ID

Sign in to HMRC's business tax account with the company's Government Gateway user ID and password, or create them from the same page if the company has none (GOV.UK: Add Corporation Tax to your business tax account). Check whether the company already has a user ID before creating another. Each sign-in also asks for an access code sent by text message, voice call or authenticator app (GOV.UK: Help signing in to HMRC online services), which is not the activation code.

Step 3: Enrol for Corporation Tax with the company's details

Select "Services you can add", find the Corporation Tax service and select "Enrol for service" (GOV.UK: Add Corporation Tax to your business tax account). Have ready the company registration number, the UTR, the date the company started to do business and the date its first accounts are made up to. The law gives a company 3 months from the start of its first accounting period to tell HMRC (legislation.gov.uk: FA 2004 s.55), so a company that has started trading should enrol straight away.

Step 4: Watch for the activation code at the registered office

HMRC posts the activation code, instructions for activating the service and the company's Corporation Tax payment deadline to the registered office, within 10 days or 21 if you live abroad (GOV.UK: Add Corporation Tax to your business tax account). HMRC's help says the envelope is marked "Government Gateway" (HMRC: sign-in and registration FAQs). If the registered office is an accountant's or formation agent's address, the letter goes there, so ask them to forward it the day it arrives.

Step 5: Activate Corporation Tax within 28 days

Sign back in with the same user ID and follow the letter's instructions to enter the activation code. HMRC's help says you must activate within 28 days of the date on the letter, or the code expires and you have to request a new one (HMRC: sign-in and registration FAQs). Once the code is accepted, the company's user ID and password are its login for Corporation Tax online, including filing the return.

Step 6: Request the Companies House authentication code

If the company doesn't hold its 6-character authentication code, request it through Companies House WebFiling, which you sign in to with GOV.UK One Login rather than the HMRC user ID (GOV.UK: Filing your Companies House information online). Companies House posts the code to the registered office, which can take up to 10 working days, or to your home address if you can't reach the registered office (GOV.UK: Company authentication codes for online filing). You need it to file accounts, which are usually due at Companies House 9 months after the financial year ends, 3 months before the CT600 (GOV.UK: Accounts and tax returns for private limited companies).

Step 7: File the CT600 with the company's own login

When the accounts and return are ready, file through software, because HMRC's own online filing service closed on 31 March 2026 (GOV.UK: Closure of HMRC's file-your-accounts-and-tax-return service). At submission, enter the company's Government Gateway user ID and password, the ones now enrolled for Corporation Tax. The guide to filing a CT600 yourself covers the return itself, which is due 12 months after the accounting period ends (GOV.UK: Company Tax Returns: Overview).

Step 8: Check HMRC accepted the return

Check that the software shows HMRC's acceptance, not just "sent", and keep the acknowledgement. If the submission is refused because of the login, sign in to the business tax account and confirm that Corporation Tax is listed as a service you can use (GOV.UK: Sign in to your HMRC business tax account). The after-filing checklist covers what to keep and when the tax is due.

When should you start if your deadline falls in 2026 or 2027?

Enrol for the HMRC activation code at least 34 days before the CT600 deadline (56 if you live abroad), and request the Companies House code at least 20 working days plus 14 days before the accounts deadline. Those are Taxley's own planning margins, not HMRC rules: each allows GOV.UK's stated delivery time twice, plus 14 days to file (GOV.UK: Add Corporation Tax to your business tax account).

Latest safe start dates, worked back from each deadline (Taxley's planning margin, not an HMRC or Companies House rule; working days skip weekends and England and Wales bank holidays)

Taxley's method starts from each deadline, keeps 14 days to activate, prepare and file, then allows the stated delivery time twice, so a lost or expired letter can still be replaced. The delivery times are GOV.UK's: up to 10 days for the HMRC code, or 21 abroad (GOV.UK: Add Corporation Tax to your business tax account), and up to 10 working days for the Companies House code (GOV.UK: Company authentication codes for online filing). GOV.UK gives no time for a copy of a lost UTR, so the margin borrows its 15 working days for a new company's UTR. The dates assume a 12-month period that isn't the company's first; for any other dates, count back the same way from the deadlines the deadline calculator gives.

What if the activation code is lost or has expired?

Request a new one. GOV.UK says you can ask for a new activation code if the first doesn't arrive or has expired (GOV.UK: Add Corporation Tax to your business tax account), and HMRC's help says a code expires 28 days after the date on its letter (HMRC: sign-in and registration FAQs). The new code also comes by post, so enter it the day it arrives.

Before asking again, check where the first letter went, because a replacement goes to the same place. GOV.UK says the registered office is where HMRC sends its Corporation Tax letters, and that you must tell Companies House if that address needs updating (GOV.UK: Add Corporation Tax to your business tax account). If the registered office is a former accountant's address, update it before you request anything. Companies House can post its authentication code to your home address instead, but won't email the code or read it out over the phone (GOV.UK: Company authentication codes for online filing). Each replacement HMRC code restarts a wait of up to 10 days, which is why Taxley's start-by dates allow the delivery time twice.

Can you file with your accountant's login instead?

No. An accountant files through the practice's own agent account, using authority the company gave it. GOV.UK tells clients not to give their sign-in details to an agent, and tells agents not to use a client's (GOV.UK: Change or remove your tax agent's authorisation). To file yourself, the company needs its own user ID with Corporation Tax activated.

You don't have to remove the accountant to file yourself: HMRC's business tax account guide says a company that uses an agent can still use its own account and add services to it, and should use only one Government Gateway user ID (HMRC: How to use your HMRC business tax account). When the relationship has ended, remove the authorisation in the business tax account: "Manage account", then "Add, view or change tax agents", Corporation Tax, "manage agents" and "remove" (GOV.UK: Change or remove your tax agent's authorisation). You can also write to HMRC's Central Agent Authorisation Team, quoting the company's UTR. If the accountant set up the company's own user ID, ask for it rather than creating a second one, and change the password.

The same handover applies at Companies House. A former accountant may still know the company's authentication code, and Companies House says to change the code in that case and to tell anyone else who files for you, because the old code then stops working (GOV.UK: Company authentication codes for online filing). The accountant handover checklist lists the records to collect at the same time, such as last year's filed return and the schedules that carry forward into the next return. If the company's affairs have grown complicated, paying an accountant can still be the better choice, and do I need an accountant? sets out when one is worth it.

How does Taxley use the company's login and codes?

Taxley asks for the company's own Government Gateway user ID and password when you submit the CT600 and uses them for that submission only; it doesn't keep the password. It files the accounts with Companies House using the company's authentication code, which you save once on the company page.

Taxley builds the accounts and tax computation in iXBRL, the format HMRC requires for online returns (COM60040), and sends them with the CT600 using the login you enter at submission. The form asks for a Government Gateway user ID and password; it has no GOV.UK One Login option. The password travels to the submission encrypted and is blanked out of the copy of the submission Taxley keeps, which is stored encrypted and still shows the user ID. The authentication code is kept, because every Companies House filing needs it: Taxley stores it encrypted and never shows it again. An accountant filing through Taxley uses the practice's HMRC agent login instead. To see whether your company's return is one Taxley covers, take the 30-second check to see whether Taxley fits your company.

Frequently asked questions

How long does the Corporation Tax activation code take to arrive?

GOV.UK says HMRC sends it within 10 days, or 21 days if you live abroad, by post to the company's registered office. HMRC's help pages say to use it within 28 days of the date on the letter, or it expires.

Is the Companies House authentication code the same as the HMRC activation code?

No. The HMRC activation code switches Corporation Tax on in the company's Government Gateway account. The 6-character Companies House authentication code authorises the company's filings at Companies House, and Companies House says a Government Gateway account can't be used to file with it.

Can I register for Corporation Tax online without a UTR?

No. Enrolling for Corporation Tax asks for the company's 10-digit UTR. If it hasn't arrived 15 working days after the company was registered, request a copy online. HMRC posts it to the address held by Companies House rather than emailing it.

Can I use GOV.UK One Login for the company's business tax account?

Not on GOV.UK's current guidance: its page on adding Corporation Tax asks for a Government Gateway user ID. HMRC began offering One Login to new individual customers on 9 February 2026. Existing users keep their Gateway ID until HMRC contacts them, even if they use One Login for Companies House.

Does a company that isn't trading need to add Corporation Tax yet?

Usually not. GOV.UK says a company that isn't doing business is usually dormant for Corporation Tax. Once it starts buying, selling, advertising, renting property or employing someone, add Corporation Tax: the law gives 3 months from the start of its first accounting period to tell HMRC.

General information, not personalised tax advice. Delivery times, deadlines and bank holidays checked on GOV.UK on 26 September 2026.