CT600 Filing Agent Authorisation with HMRC

If someone else files your company's CT600 — whether that's an accountant, tax adviser, or filing service — they need to be authorised by HMRC as your agent. Without authorisation, they can't submit your return or access your corporation tax records.

This guide explains exactly how agent authorisation works, how to set it up, and what to do if it goes wrong.

What Is Agent Authorisation?

Agent authorisation is HMRC's system for letting a third party act on your company's behalf for tax matters. Once authorised, your agent can:

File your CT600 and supplementary pages
Submit iXBRL accounts to HMRC
View your corporation tax position — liabilities, payments, deadlines
Correspond with HMRC about your corporation tax affairs
Amend previously filed returns

The authorisation is specific to corporation tax — it doesn't automatically give the agent access to your VAT, PAYE, or other tax affairs (those require separate authorisations).

Do You Need Agent Authorisation?

Not always. It depends on how your CT600 is being filed:

You DO need authorisation if:

An accountant or tax adviser files the CT600 using their own HMRC credentials
A filing service submits the return through its own agent account with HMRC
Anyone other than you needs to view or manage your corporation tax position online

You DON'T need authorisation if:

You're filing the CT600 yourself using your company's own Government Gateway credentials
You use software (like Taxpipe) that files through your company's Government Gateway account rather than an agent account
Someone prepares your accounts but you submit the CT600 yourself

Taxpipe note: Taxpipe files your CT600 directly through your company's Government Gateway account. You enter your credentials, and we submit on your behalf. This means you don't need to set up agent authorisation — it's simpler and faster.

How Agent Authorisation Works: The Process

There are two main ways to authorise an agent for corporation tax:

Method 1: Online Authorisation (Recommended)

This is the faster method. Here's the step-by-step:

Your agent does this:

Logs into their HMRC agent account (Agent Services Account)
Requests authorisation for your company
Provides your company's UTR (Unique Taxpayer Reference) and company name
HMRC generates an authorisation link or reference

You (the company) do this:

Receive the authorisation request (by email or from your agent)
Log into your company's Government Gateway account
Accept the authorisation request
Confirm the agent's details

The online process usually completes within 24-48 hours.

Method 2: Paper Authorisation (Form 64-8)

If online authorisation isn't possible, you can use HMRC form 64-8:

Download form 64-8 from GOV.UK
Complete the form with your company details:
- Company name and address
- UTR number
- Company Registration Number
- The agent's details (name, address, agent reference)
The form must be signed by a company officer (director or company secretary)
Post it to the relevant HMRC office

Processing time: 2-4 weeks by post. This is significantly slower than online, so use it only as a fallback.

What Your Agent Needs from You

To set up authorisation, your agent will typically need:

Information	Where to Find It
Company UTR	HMRC welcome letter, previous CT600, or your HMRC online account
Company Registration Number	Companies House certificate or confirmation statement
Company name (as registered)	Companies House
Registered office address	Companies House
Government Gateway user ID	The credentials you use to log into HMRC services
Agent's reference number	Your agent will have this

Can't find your UTR? Read our guide on the CT600 and UTR numbers. You can also call HMRC's Corporation Tax helpline on 0300 200 3410 to request it.

Authorising Multiple Agents

Your company can have multiple agents authorised for corporation tax at the same time. This is useful if:

You have one accountant for compliance and another for advisory work
You're transitioning from one accountant to another
A filing service handles the CT600 while your accountant advises on tax planning

Each agent is authorised independently. Authorising a new agent doesn't automatically remove an existing one.

Removing an agent

You can revoke an agent's authorisation at any time:

Log into your Government Gateway account
Go to your corporation tax service
Find the agent in your authorised agents list
Remove their authorisation

Your agent can also remove themselves if they no longer act for you.

Changing Accountants: Authorisation Handover

Switching from one accountant to another is a common scenario. Here's how to handle the authorisation:

Step 1: Authorise your new agent

Follow the online process above with your new accountant's details.

Step 2: Remove your old agent (optional but recommended)

Once you've confirmed the new accountant is set up, remove the old agent's authorisation. While they can't do anything harmful with it (they need your Government Gateway credentials to file), it's good practice.

Step 3: Transfer records

Your old accountant should provide:

Previous year's CT600 and accounts
Corporation tax computations
HMRC correspondence
Your UTR and Government Gateway details
Details of any outstanding matters

Note: Professional accounting bodies require outgoing accountants to cooperate with handovers. If your old accountant is being difficult, contact their professional body.

Common Problems and How to Fix Them

"The authorisation request has expired"

Online authorisation requests expire after 10 days. If you don't accept in time, your agent needs to send a new request.

"I can't find the authorisation request"

Log into your Government Gateway account and check for pending requests. Make sure you're logged in with the correct credentials for the company (not your personal tax account).

"The UTR doesn't match"

Ensure you're using the corporation tax UTR (10 digits), not the Self Assessment UTR. They're different numbers even if you're also registered for Self Assessment.

"Agent reference number not recognised"

Your agent's reference must be registered with HMRC for corporation tax. New agents who recently registered may need to wait for HMRC to process their application.

"My Government Gateway account is locked"

If you've forgotten your credentials or they've been locked, you'll need to recover or reset them. See our Government Gateway guide.

"Authorisation was removed without my knowledge"

This can happen if HMRC detects an issue or if someone with access to your Government Gateway account made changes. Contact HMRC to investigate and reauthorise your agent.

Agent Authorisation for Groups

If your company is part of a corporate group, each company in the group needs separate agent authorisation. There's no "group-wide" authorisation.

This means:

Each subsidiary needs its own authorisation form or online request
The agent needs each company's individual UTR
Each company's directors (or parent company officers with authority) must approve

For large groups, this can be time-consuming. Start the process early, especially when appointing a new group auditor or tax adviser.

Group relief claims and group loss relief submissions require the agent to be authorised for both the surrendering and claimant companies.

DIY Filing vs Agent Filing

You don't need an agent to file your CT600. Many small companies choose to file themselves using software.

Advantages of filing yourself:

No agent authorisation needed — simpler setup
Direct control — you see exactly what's being filed
Cost savings — no accountant fees (just software costs)
Speed — no waiting for an accountant's schedule

When you should use an agent:

Complex tax affairs (multiple income types, capital gains, group structures)
R&D tax relief claims
HMRC enquiries or disputes
You don't have time or confidence to file yourself

Read our full comparison: accountant vs DIY CT600 filing.

HMRC Agent Services Account

Since 2019, HMRC has been migrating agents to the new Agent Services Account (ASA), which sits alongside the older Government Gateway agent credentials.

Your agent may use either or both:

Legacy Government Gateway agent account — still works for CT600 filing
Agent Services Account — required for Making Tax Digital services and some newer HMRC systems

For corporation tax specifically, the legacy account still works fine. However, when Making Tax Digital for corporation tax is eventually introduced, agents will need the ASA.

Security and Privacy

What can an authorised agent see?

Your corporation tax liabilities and payments
Filed CT600 returns and accounts
HMRC correspondence about corporation tax
Your company's UTR and basic details

What can't they see?

Your Government Gateway password
Other tax regimes (VAT, PAYE) unless separately authorised
Your personal tax affairs
Bank account details

Keeping your account secure

Never share your Government Gateway password with your agent — they don't need it
Review authorised agents annually — remove any you're no longer using
Enable two-factor authentication on your Government Gateway account
Check for unauthorised filings — log in periodically and review submitted returns

Frequently Asked Questions

How long does agent authorisation take?

Online authorisation takes 24-48 hours once you accept the request. Paper authorisation (form 64-8) takes 2-4 weeks. Always use online if possible.

Can my accountant file without authorisation?

Not through their agent account. However, if you give them access to your company's Government Gateway credentials, they can file as if they were you. This isn't recommended for security reasons, but it's technically possible.

Does authorising an agent give them access to my bank?

No. HMRC agent authorisation only covers tax matters. Your agent cannot access your bank accounts, Companies House filings, or anything outside HMRC's tax systems.

Can I authorise an agent if I'm not a director?

Generally, no. Agent authorisation must be approved by a company officer — typically a director. A company secretary can also authorise in some cases. An employee without officer status cannot approve the authorisation.

What happens if my agent misses a deadline?

You're still responsible. Even if your agent fails to file on time, HMRC will penalise the company — not the agent. You may have recourse against the agent separately, but HMRC penalties fall on the company.

Do I need new authorisation every year?

No. Once granted, agent authorisation continues until you or the agent revoke it. There's no annual renewal process.

File Your CT600 Without Agent Authorisation

Taxpipe lets you file your CT600 directly through your company's Government Gateway account — no agent authorisation required. Just enter your company details, answer straightforward questions about your income and expenses, and we submit your CT600 and iXBRL accounts to HMRC.

From £99. No accountant needed.

File your CT600 with Taxpipe →