Government Gateway and Corporation Tax Enrolment

To file a CT600 through TinyTax, you need to connect your HMRC Government Gateway account. This guide explains how Government Gateway works, how to enrol for Corporation Tax, and how to fix common issues.

---

What Is Government Gateway?

Government Gateway is HMRC's online login system. It's separate from Companies House — they are two different systems with different credentials:

System	What it's for	Credential
Government Gateway	HMRC services (CT600, PAYE, VAT)	12-digit Gateway ID + password
Companies House	Filing accounts, company changes	6-character authentication code

TinyTax needs your Government Gateway credentials to submit your CT600 to HMRC. Your Companies House authentication code is used separately when filing accounts.

---

Organisation vs Individual Accounts

Government Gateway has two account types. Corporation Tax can only be added to an Organisation account.

Account type	Used for	Can enrol for CT?
Individual	Personal tax (Self Assessment)	No
Organisation	Business taxes (CT, PAYE, VAT)	Yes

How to check your account type

1. Log in at gov.uk/log-in-register-hmrc-online-services
2. Look at what services are listed — if you see "Self Assessment" but not "Corporation Tax", you're on an Individual account

---

How to Enrol for Corporation Tax

Step 1: Create an Organisation Gateway account (if needed)

If you only have an Individual account:

1. Go to gov.uk/log-in-register-hmrc-online-services
2. Select Create sign in details
3. Choose Organisation when asked what type of account
4. Complete the setup — you'll receive a new 12-digit Gateway ID

Step 2: Add Corporation Tax to your account

1. Log in to your Organisation Gateway account
2. Select Add a tax, duty or scheme
3. Choose Corporation Tax
4. You'll need:

- Your Company UTR (10-digit Unique Taxpayer Reference — from HMRC's "Notice to deliver a Company Tax Return" letter) - Your company registration number (from Companies House)

After submitting the enrolment form: HMRC will post a CT Activation Code to your company's registered address. This usually arrives within 7–10 working days. Log back in to your Gateway, go to Corporation Tax, and enter the code to complete enrolment. Until this step is done, Corporation Tax won't appear as an active service and TinyTax won't be able to submit your CT600.

Step 3: Connect to TinyTax

Once Corporation Tax appears in your Gateway services:

1. In TinyTax, go to the submission preview page
2. Enter your Company UTR, Gateway ID, and Gateway password when prompted
3. TinyTax connects directly to HMRC to submit your return

---

Common Problems

"Company not listed" or "not recognised"

This usually means one of:

• You're using an Individual Gateway — Corporation Tax requires an Organisation account (see above)
• HMRC hasn't processed your CT registration yet — after incorporation, HMRC automatically registers your company for CT, but this can take several weeks. If your company is very new, you may need to wait
• Your UTR is incorrect — double-check the 10-digit number from HMRC's letter

Stuck in a loop when trying to enrol

If HMRC keeps redirecting you back to the same page:

1. Make sure you're logged into an Organisation account, not Individual
2. Try a different browser or clear your cookies
3. If the problem persists, call HMRC's Corporation Tax helpline: 0300 200 3410

CT Activation Code delayed or not arrived

When you enrol for Corporation Tax online, HMRC posts a one-time CT Activation Code to your company's registered address. You must enter this code on your Gateway to complete enrolment.

First, check if CT is already enrolled:

1. Log in at gov.uk/log-in-register-hmrc-online-services
2. If Corporation Tax appears in your list of services, enrolment is already complete — no activation code needed, and you can file through TinyTax right away

If Corporation Tax is not listed yet:

• Codes normally arrive within 7–10 working days
• Make sure you're checking the address registered at Companies House for your company — that's where HMRC posts it
• If it's been longer than 10 working days, call the HMRC Corporation Tax helpline on 0300 200 3410 and ask them to resend the code

Agent has CT access on their Gateway

If your accountant or agent has Corporation Tax registered on their Gateway with formal HMRC authorisation:

• They can file on your behalf through TinyTax — at the submission step, select Tax agent and use their agent Gateway ID and password
• This doesn't necessarily prevent you from also enrolling on your own Gateway
• If HMRC won't let you enrol while an agent has CT access, ask your agent to de-authorise their access first, then re-enrol

Multiple Gateway accounts

If you accidentally created multiple Gateway accounts:

• Only one Organisation account should have Corporation Tax enrolled
• You can call HMRC to delete unused Gateway accounts: 0300 200 3600
• Make sure you know which Gateway ID has CT enrolled before deleting any

---

Government Gateway vs Companies House Auth Code

These are commonly confused. Here's a quick comparison:

	Government Gateway	CH Auth Code
Issued by	HMRC	Companies House
Format	12-digit ID + password	6-character code
Used for	CT600 tax returns	Company accounts filing
How to get	Create account online	Sent by post to registered office
Used in TinyTax	When submitting CT600	When submitting accounts

If you're having trouble with your Companies House authentication code, see our authentication code guide.

---

Common Questions

How do I check if Corporation Tax is already enrolled on my Gateway?

Log in at gov.uk/log-in-register-hmrc-online-services. On your home screen, look for Corporation Tax in the list of services. If it's there, enrolment is complete and you can use those Gateway credentials in TinyTax straight away. If only other services (Self Assessment, PAYE, etc.) appear but not Corporation Tax, enrolment hasn't been completed yet.

I've lost my Gateway ID — how do I find it?

Check your email for the original registration confirmation from HMRC. If you can't find it, you can recover your Gateway ID at gov.uk/log-in-register-hmrc-online-services by selecting "Problems signing in" and following the recovery steps.

Do I need a Government Gateway to file accounts with Companies House?

No. Companies House accounts use a separate authentication code, not Government Gateway. You only need Gateway for your CT600 tax return.

My company is being struck off — can I still file CT600?

Yes. You should file any outstanding CT600 returns before your company is dissolved. Your Government Gateway access remains active until HMRC closes the Corporation Tax record after dissolution.

Can I use my accountant's or agent's Gateway to file through TinyTax?

Yes — if your accountant or agent has formal HMRC agent authorisation for your company. When submitting, they select Tax agent instead of "Company director" at the filing step, then enter their agent Gateway ID and password.

This requires prior agent authorisation from HMRC — either via a signed 64-8 form or the online agent authorisation process. Without that authorisation, HMRC will reject the submission.

If neither the company nor an authorised agent has Gateway credentials, the company will need to create their own Organisation Gateway account and enrol for Corporation Tax (see the steps above).

I've been filing CT600s directly on HMRC's website for years — do I still need to set this up?

If you previously filed CT600s through HMRC's own online portal, that service closed on 31 March 2026. From April 2026, all CT600 returns must be filed through commercial software.

The good news is you almost certainly already have a Government Gateway account — your previous HMRC filings would have required one. Log in at gov.uk/log-in-register-hmrc-online-services and check whether Corporation Tax appears in your list of services.

• If Corporation Tax is listed: enrolment is already complete. Use your existing 12-digit Gateway User ID and password when submitting through TinyTax — no further setup needed.
• If Corporation Tax isn't listed: you'll need to enrol. Follow the steps under "How to Enrol for Corporation Tax" above. This is a one-time process.

Note: having MTD VAT set up on your Gateway does not automatically mean Corporation Tax is enrolled — they are separate enrolments and may be on different account types.

Why does TinyTax need my Gateway password? Is it stored?

TinyTax asks for your Gateway password because HMRC's CT600 API (the Transaction Engine) requires credentials to be included in each submission request. This is HMRC's design — they provide credential-based authentication for corporation tax filing, not an OAuth or delegated-access flow. All commercial CT600 software (Xero, FreeAgent, etc.) works through this same mechanism.

Your password is never stored:

• Not saved to our database
• Not written to any log files
• Not retained in any form after the submission request completes

It exists only in memory during the API call to HMRC, then it's gone. You can verify this: after filing, there's no way to retrieve or view the password you entered, because we don't hold it.

What about HMRC's "never share your password" warning?

That guidance targets phishing and fraud — criminals who ask you to hand over credentials so they can log in to your HMRC account directly. Filing through commercial software is a different mechanism: your credentials are submitted to HMRC's own API to authenticate your return, not used to access your online tax account. HMRC explicitly supports this filing route as the authorised alternative to their now-closed direct filing service.

If you're uncomfortable entering your password, the only alternative is to use a tax agent who has formal HMRC agent authorisation for your company — they can file using their own agent credentials via the Tax agent option at the submission step.