- Corporation Tax filing is done by submitting a Company Tax Return, also known as a CT600, to HMRC.
- A complete Company Tax Return includes the CT600, statutory accounts, and tax computations in iXBRL format.
- The filing deadline is 12 months after the end of your accounting period, even if your company made a loss.
Corporation Tax filing is a required part of running a UK limited company. Each year, you must report your profits to HMRC by submitting a Company Tax Return, even if your company made a loss or has no tax to pay. Missing requirements or deadlines can lead to penalties and unnecessary stress.
In practice, Corporation Tax filing involves submitting a CT600 alongside your company accounts and tax computations, all filed digitally with HMRC. Understanding how these documents fit together is especially important for first-time directors.
This guide explains what Corporation Tax filing involves, who must submit a Company Tax Return, what documents are needed, as well as how expert accounting services can help you more than you might think.
What does Corporation Tax filing actually involve?
Corporation Tax filing means submitting a Company Tax Return to HMRC for each accounting period. Although many directors call this “filing Corporation Tax”, the formal process is the CT600 submission with supporting documents.
Importantly, this filing tells HMRC how much Corporation Tax your company owes. Only after the return is submitted can HMRC finalise your liability, even if the amount due is zero.
In practice, filing covers three core elements, all submitted together online.
What is a Company Tax Return (CT600)?
A Company Tax Return is the official way your company reports its profits, losses, and adjustments for Corporation Tax. The central document is the CT600 form, but it never stands alone.
Instead, HMRC expects the CT600 to be submitted alongside your accounts and tax computations. Together, these documents explain how your taxable profit was calculated and why that figure differs from your accounting profit.
As a result, Corporation Tax filing is as much about accurate records as it is about meeting deadlines.
Although your Company Tax Return isn’t due until 12 months after your accounting period, your Corporation Tax payment is usually due much earlier. Preparing your CT600 filing well in advance gives you time to review adjustments, claim reliefs, and avoid last-minute errors. It also ensures you’re not caught out by a tax bill you weren’t expecting when it’s time to pay.
What documents do you need to file Corporation Tax?
To complete Corporation Tax filing correctly, HMRC requires a full digital submission. Before you start, make sure you have the following ready.
You must submit:
- A completed CT600 Company Tax Return
- Statutory company accounts for the period
- Corporation Tax computations explaining adjustments
- Any supplementary pages for reliefs or special claims
All documents must be filed in iXBRL format. Therefore, most companies use accounting or tax software rather than manual forms.
Who needs to file a Company Tax Return?
Most UK limited companies must file a Company Tax Return for every accounting period. This applies whether the company made a profit, broke even, or reported a loss.
However, if your company is dormant for Corporation Tax purposes, HMRC may not require a return. That said, HMRC and Companies House use different definitions of “dormant”, which often causes confusion.
To avoid penalties, it is essential to confirm your status early. Our guide to trading vs non-trading for Corporation Tax explains how HMRC decides whether a return is required.
How do you register for Corporation Tax filing?
Before you can file Corporation Tax, your company must be registered with HMRC for Corporation Tax. This usually happens automatically after incorporation, but it is not guaranteed in all cases.
You must register within three months of starting to trade. Trading includes issuing invoices, earning income, or advertising your services.
You can register directly with HMRC using their official online service. Use HMRC’s official Register for Corporation Tax page to complete this step correctly.
Once registered, HMRC issues your company with a Unique Taxpayer Reference, which you need for every CT600 submission.
How do you file a Company Tax Return online?
Almost all Corporation Tax filing is done online. Paper CT600 submissions are only allowed in very limited circumstances, such as accessibility exemptions.
To file online, you must use compatible commercial software that supports iXBRL. This software submits your CT600, accounts, and computations together in one package. HMRC no longer provides a free online CT600 filing service, and iXBRL digital filing has been mandatory for most companies for some time.
Some future changes around digital reporting, including discussions about Making Tax Digital for Corporation Tax, are ongoing. For more on how digital tax reporting is evolving, see our guide on MTD for Corporation Tax — but currently this requirement does not yet apply.
When is the Corporation Tax filing deadline?
The filing deadline for Corporation Tax is separate from the payment deadline. This often catches directors out.
Your Company Tax Return must be filed within 12 months of the end of your accounting period. For example, if your year end is 31 March 2026, your CT600 must be filed by 31 March 2027.
However, Corporation Tax payment is due earlier. Our guide on how to pay Corporation Tax explains how the two deadlines interact.
What happens if you file Corporation Tax late?
Late Corporation Tax filing triggers automatic penalties from HMRC. These penalties apply even if no tax is due.
The standard penalties are:
- £100 if your return is one day late
- Another £100 if it is three months late
- 10 percent of unpaid tax after six months
- A further 10 percent after twelve months
Repeated late filing increases penalties significantly. For a wider compliance overview, see our guide on HMRC and Companies House fines.
What if your company made a loss or has no tax to pay?
Even if your company made a loss, Corporation Tax filing is still required. HMRC will not assume a nil liability unless you submit a return.
Filing during loss-making periods is also important because losses can often be carried forward or back to reduce future tax bills. Skipping a return may mean losing access to that relief.
Therefore, filing on time protects both compliance and future tax efficiency.
How does first-year Corporation Tax filing work?
First-year Corporation Tax filing is slightly more complex. Your first set of accounts may cover more than 12 months, but a CT600 can only cover up to 12 months.
As a result, many new companies must file two Company Tax Returns in their first year. After that, accounting periods usually align automatically.
Planning ahead in your first year avoids rushed filings and duplicated work later.
How Sleek helps you with Corporation Tax filing
Corporation Tax filing involves more than completing a form. Between preparing compliant accounts, calculating adjustments, and submitting everything digitally, the process can quickly become overwhelming.
Sleek handles the full filing process for you, from preparing your CT600 to submitting compliant iXBRL accounts and tax computations. Everything is reviewed by experienced accountants, so deadlines are met and penalties are avoided.
With clear timelines and ongoing support, you stay compliant without losing focus on running your business.
Disclaimer: The preceding information is not legal advice. This content is aimed to provide general guidance. For more formal or legal advice, contact Sleek directly.
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FAQs on corporation tax filing
How do I file a Corporation Tax return in the UK?
You file Corporation Tax by submitting a Company Tax Return to HMRC. This includes a CT600 form, your company accounts, and Corporation Tax computations. Most companies must file online using iXBRL-compatible software. HMRC no longer accepts standard paper returns, so digital filing is now the default for compliance.
What is the deadline for Corporation Tax filing?
The deadline for Corporation Tax filing is 12 months after the end of your accounting period. This deadline applies even if your company made a loss or has no tax to pay. Importantly, this filing deadline is later than the Corporation Tax payment deadline, which usually comes first.
Where do I file my Corporation Tax return?
You file your Corporation Tax return directly with HMRC, not Companies House. The submission is made through approved commercial software that sends your CT600, accounts, and tax computations together. Companies House only receives your statutory accounts, which is a separate filing obligation.
What form does a company use to file Corporation Tax?
UK companies file Corporation Tax using form CT600. However, the CT600 is only one part of the full Company Tax Return. You must also submit your statutory accounts and Corporation Tax computations, all in iXBRL format, for HMRC to accept the filing as complete.
Do I still need to file Corporation Tax if my company made a loss?
HMRC accepts corporate credit cards, but there is a non-refundable fee for this service. Personal credit cards are not accepted, though you can use a personal debit card without incurring a fee.
Can I get an extension to file Corporation Tax?
HMRC does not normally grant extensions for Corporation Tax filing. If you miss the deadline, automatic penalties apply, even if no tax is due. Only exceptional circumstances, such as serious illness or system failures, may be accepted as a reasonable excuse for late filing.
When do I file my first Corporation Tax return?
Your first Corporation Tax return is usually due 12 months after the end of your first accounting period. However, because first-year accounts can cover more than 12 months, many new companies must file two Company Tax Returns in their first year before dates align going forward.
