- You must tell HMRC your company is active within 3 months of trading.
- You register online via the Government Gateway using your company UTR.
- Filing a company tax return late costs £200 from day one, even with no tax due.
To register for corporation tax you must tell HMRC your company is active within three months of starting to trade, and you do it online through the Government Gateway using your company’s Unique Taxpayer Reference. If you have just started trading and want a tax accountant to handle corporation tax for you, that clock has probably already started. Starting to trade covers buying stock, advertising, employing someone or renting premises. Incorporating the company on its own does not start it.
Registration is a separate step from setting the company up at Companies House. Companies House tells HMRC your company exists, but that notification does not register you for the tax. You still have to do it yourself.
Once you register, HMRC issues a notice to deliver a company tax return and confirms your accounting period, which is what your filing and payment deadlines are then measured from.
When does the three-month clock start?
The deadline runs from the day your company starts trading, not the day it was incorporated. That distinction catches a lot of business owners out, because the two dates are often weeks or months apart.
So a company formed in January that does not take on its first client until May has until August to register, three months from May. If you incorporated and started trading on the same day, the clock started on that day.
HMRC’s own guidance is that you register within three months of your company becoming active. If you are late, you may get a penalty, so the safe approach is to register as soon as you know trading has begun rather than waiting for the exact three-month edge.
What counts as starting to trade?
This is the part most guides skip, and it is where the confusion sits. HMRC treats your company as active once it starts doing business, which is a lower bar than making a profit or even sending your first invoice.
Activities that count as starting to trade include:
- Buying stock or raw materials to sell on
- Advertising or actively marketing for customers
- Employing someone
- Renting business premises
- Receiving taxable income, such as your first paid contract or bank interest on company funds
Groundwork usually does not count. Registering a domain, building a website, designing a logo or writing a business plan while you get ready to launch is normally pre-trading, not trading. The line gets crossed once money starts moving in either direction for actual business activity.
If you are genuinely unsure whether your company has crossed into trading, the safe move is to treat the earliest of those activities as your start date and register from there.
Where does your company UTR come from?
You need your company’s Unique Taxpayer Reference before you can register online. This is a 10-digit number HMRC uses to identify your company on every tax return and payment.
HMRC posts the UTR to your registered office shortly after incorporation, usually within a couple of weeks. It often arrives as part of a letter, sometimes referred to as form CT41G. Your company UTR is not the same as your personal UTR for self assessment, so keep the two separate.
If the UTR has not turned up and you are approaching the deadline, do not wait for the post. You can request it through your HMRC online account, or call the corporation tax helpline on 0300 200 3410 to have it confirmed or reissued.
How to register, step by step
You register for corporation tax online through HMRC. Have the following to hand before you start:
- Your company’s 10-digit UTR
- Your company registration number from the certificate of incorporation
- The date your company started trading
- The date your annual accounts are made up to (your accounting reference date)
With those ready, the process runs as follows:
- Sign in to HMRC online services with your Government Gateway user ID. If you do not have one, set it up first, as you cannot register without it.
- Select the option to register for corporation tax and enter your company UTR and registration number.
- Give HMRC the date your company started trading and your accounting period dates.
- Submit. HMRC then opens your corporation tax record and sets your filing and payment deadlines from the accounting period you have given.
Your accountant can register on your behalf if you would rather not deal with HMRC’s online services directly. If you already work with an accountant for a limited company, this is the kind of thing they handle as a matter of course.
What happens after you register?
Once you are registered, HMRC confirms your accounting period. This is the period your company tax return and corporation tax bill are calculated for, and it usually lines up with your financial year.
HMRC then issues a notice to deliver a company tax return. That notice is your formal instruction to submit a CT600 covering the accounting period. It is worth understanding what filing the corporation tax return involves early, because the deadline arrives sooner than most people expect.
Two separate deadlines follow, and they do not fall on the same day. The payment deadline comes first, nine months and one day after your accounting period ends. The filing deadline for the return itself is 12 months after the period ends. When the bill is due, here is how to pay the bill once it is due.
The rate you will pay is 19% on profits up to £50,000 and 25% on profits over £250,000, with marginal relief in between. We cover the current corporation tax rate in full separately, so we will not repeat it here.
What happens if you register late?
Registering late for corporation tax can trigger a penalty from HMRC, and the bigger consequence usually comes at the filing stage. HMRC charges fixed penalties for filing your company tax return after the deadline, and these apply even if your company owes no tax at all.
From 1 April 2026 the fixed late-filing penalties doubled. The current schedule for a late company tax return is:
| Time after the filing deadline | Penalty |
|---|---|
| 1 day late | £200 |
| 3 months late | Another £200 |
| 6 months late | HMRC estimates your bill and adds 10% of the unpaid tax |
| 12 months late | Another 10% of the unpaid tax |
If your return is late three times in a row, the £200 fixed penalties rise to £1,000 each. HMRC and Companies House run separate penalty regimes, so a missed year end can bring HMRC and Companies House penalties from both bodies at the same time.
If you think you are already past the deadline, the answer is to register and file as soon as you can rather than leave it, since the fixed penalties escalate and the tax-geared ones only start once tax goes unpaid. You can appeal a late-filing penalty within 30 days if you have a reasonable excuse.
What if my company is not trading yet?
If your company is genuinely dormant, meaning it has never traded and has no income, you do not need to register for corporation tax yet. HMRC may still write to you after incorporation to confirm the company’s status, and you should tell them it is dormant.
A dormant company still has separate obligations at Companies House, and those do not go away just because there is no corporation tax to worry about. The detail sits in our guide to dormant company filing obligations.
How Sleek helps with registering for corporation tax
Registering for corporation tax is the first fixed deadline your new company has to meet, and it is closely followed by a return and a payment date that most first-time directors underestimate. Sleek’s in-house qualified accountants register your company, track every deadline from the accounting period onwards, and file the return, so a missed date is not something you have to hold in your head.
Ready to have your corporation tax position reviewed before the next deadline?
FAQs on register for corporation tax
How about corporation tax filing after I register?
After you register, HMRC issues a notice to deliver a company tax return, known as a CT600. You file it within 12 months of your accounting period end, and pay any corporation tax due nine months and one day after that period ends. The payment deadline comes before the filing deadline, which surprises a lot of first-time directors.
Do I need to register for corporation tax if my company is dormant?
No. A company that has never traded and has no income does not need to register for corporation tax. You should still respond to any letter HMRC sends after incorporation to confirm the company is dormant, otherwise HMRC may issue a notice to file expecting a return.
Is my company UTR the same as my personal UTR?
No. Your company has its own 10-digit UTR, issued by HMRC after incorporation, and it is different from any personal UTR you hold for self assessment. Use the company UTR for corporation tax registration, returns and payments.
What happens if my UTR has not arrived?
HMRC posts the company UTR to your registered office, usually within a couple of weeks of incorporation. If it has not arrived and your deadline is close, request it through your HMRC online account or call the corporation tax helpline on 0300 200 3410 rather than waiting for the letter.
Can my accountant register on my behalf?
Yes. An accountant with authorisation to act for your company can complete corporation tax registration for you through their HMRC agent account. Many business owners hand this over along with the return itself, so registration and the first CT600 are handled together.
Does registering for corporation tax also register me for VAT or PAYE?
No. Corporation tax, VAT and PAYE are separate registrations with HMRC. Registering for corporation tax does not sign you up for the others, so you register for VAT once your taxable turnover passes £90,000, and for PAYE before your first payday if you take on staff.
How long does registration take?
The online registration itself takes a few minutes once you have your company UTR and details to hand. The bigger wait is usually for the UTR to arrive by post after incorporation, which is typically a couple of weeks. Start as soon as your UTR turns up so the three-month deadline is not a concern.
