Corporation Tax can be tricky to navigate in the UK, especially if you have recently started a business or expanded into the UK from the US, the EU, or another country. If your company already files Company Tax Returns, you may also be aware that the joint online filing service for filing company accounts and Company Tax Returns is closing on 31 March 2026. Up to and including that date, companies can still use the service to file with HMRC and Companies House. From 1 April 2026, you will need to use commercial software for Company Tax Returns, while annual accounts will still need to be filed separately with Companies House.
With that in mind, it is worth understanding exactly what needs to be filed, when the deadlines apply, and what can happen if you miss them.
What to file for UK Corporation Tax
Most limited companies in the UK must file a Company Tax Return with HMRC if they receive a notice to deliver one. In practice, this usually means submitting a CT600, together with supporting accounts and a Corporation Tax computation using compatible software. From 1 April 2026, HMRC’s old joint online filing service will no longer be available, so businesses will need to use commercial software for this process.
If you are a sole trader, you do not pay Corporation Tax. Instead, your business profits are usually reported through Self Assessment. That is one reason it is important to understand the difference between a limited company and a sole trader setup before you start filing. If you are unsure which rules apply to you, U&W’s Corporation Tax and Personal Tax support can help you clarify your position.
It is also worth keeping accurate records throughout the year. Good bookkeeping makes it much easier to prepare your return correctly, while cloud systems such as Xero accounting can help keep your figures organised and up to date.
When to file: understanding your accounting period
The filing deadline for a Company Tax Return is 12 months after the end of the accounting period it covers. However, the deadline to pay Corporation Tax is usually earlier: 9 months and 1 day after the end of the accounting period for companies with taxable profits of up to £1.5 million. That means many companies must pay before they actually file the return.
Your accounting period for Corporation Tax is usually linked to your company’s financial year, but it is not always as simple as using the calendar year. In some cases, especially in your first year of trading or where your accounts cover more than 12 months, you may need to deal with more than one accounting period. HMRC says an accounting period for Corporation Tax cannot be longer than 12 months, so if your accounts run for a longer period, you must file 2 returns to cover it.
For example, if your company’s accounting period ends on 31 December 2025, your Company Tax Return is normally due by 31 December 2026. If your taxable profits are up to £1.5 million, the Corporation Tax itself would usually need to be paid by 1 October 2026.
If you change your year end or your accounts cover more than 12 months, HMRC may need your accounting period dates updated. If you do not update them properly, you may still face late filing penalties.
The end of the old online filing service
One of the biggest practical changes for small companies is the closure of the joint HMRC and Companies House online filing service on 31 March 2026. After that date, you will no longer be able to use it to file annual accounts and Company Tax Returns together. You should also save copies of past returns before the service closes, because HMRC states that previous returns will no longer be accessible through that system after 31 March 2026.
This is a good time to review your systems and deadlines. Using management accounts and up-to-date software can make it much easier to keep track of upcoming obligations and avoid a last-minute scramble.
Missed deadlines and their consequences
If you miss the filing deadline for your Company Tax Return, HMRC will charge a late filing penalty, even if there is no Corporation Tax to pay. The rules are becoming stricter for returns due on or after 1 April 2026. Under the updated penalty regime, the initial late filing penalty rises from £100 to £200, and the penalty for being more than 3 months late rises from £200 to £400. For repeated failures across 3 successive returns, the penalties increase further.
If your return is more than 6 months late, HMRC can issue a tax determination, which is its own estimate of the Corporation Tax due. HMRC states that you cannot appeal against the determination itself, although once you file the outstanding return, HMRC will recalculate the tax, interest and penalties based on the actual figures.
There is also the separate issue of late payment. Even if you file on time, missing the payment deadline can lead to interest and additional cost. This is why many directors use a Corporation Tax calculator during the year and combine it with regular reporting, rather than waiting until the filing deadline approaches.
Can you appeal a late filing penalty?
Yes, but only in certain circumstances. HMRC allows appeals where you have a reasonable excuse. The appeal must be completed in one go, and HMRC says you must file your Company Tax Return before appealing the penalty.
A reasonable excuse is generally something that stopped you meeting your tax obligation despite taking reasonable care. Serious illness, bereavement, or major unexpected events may qualify depending on the facts. Simply forgetting, misunderstanding the rules, or making an avoidable error is much less likely to succeed.
If you are struggling with deadlines because your records are behind, getting your bookkeeping and payroll processes in order can make future filings much smoother.
How to stay on top of Corporation Tax deadlines
The simplest way to avoid penalties is to build your compliance process around the earlier payment deadline, not just the filing deadline. Many companies get caught out because they focus on the CT600 deadline and forget that the tax itself is usually due 3 months earlier. HMRC’s standard rule is 9 months and 1 day for payment, then 12 months for filing.
A practical approach is to:
- Keep records updated every month
- Review profit levels regularly
- Estimate your Corporation Tax before year end
- Set reminders for both the payment deadline and the return deadline
- Use approved software rather than leaving everything until the last moment
Final thoughts
Corporation Tax deadlines in the UK are straightforward once you separate the key obligations. In most cases, you need to pay Corporation Tax 9 months and 1 day after the end of the accounting period, and file the Company Tax Return 12 months after the end of that period. From 1 April 2026, you will also need to use commercial software because the old joint filing service is closing.
Missing deadlines can become expensive very quickly, especially with the increased late filing penalties applying to returns due from 1 April 2026. If you want to avoid penalties, reduce admin stress and keep everything filed correctly, U&W’s Corporation Tax, Xero accounting, and management accounts services can help you stay on top of your obligations all year round.
