CT600 Filing in the UK

When starting and operating a limited company in the UK, there are numerous responsibilities to consider, and one of the main tasks is that of submitting your corporation’s tax return to the HMRC annually. Such a return is filed on a form called the CT600, and whether your company has been a healthy profit maker, has broken even, or is still finding its feet, filing it correctly and on time is not an option.

The good news is that CT600 filing in the UK is much easier than it sounds when you know what it entails, what the deadlines are and what you can claim to reduce your tax bill. The guide will contain all that startup founders and business owners must know about the filing of HMRC CT600 in the year 2026. The guide is written with correct rates and updated rules throughout the document.

What Is CT600 Filing in the UK?

The CT600 is the annual company tax return form that every active limited company in the UK must complete and send to HMRC. Making it correctly will make sure you pay the correct amount of tax, utilise all reliefs available, and do not make any under- or overpayments.

Think of it as your company’s annual tax report. It reports to HMRC the amount of profit your business made in the accounting period, the reliefs and deductions you are claiming and how much corporation tax you owe as a result. The CT600 return in the UK is not a standalone document; it has to be shared along with your statutory accounts and tax computations in one of the digital formats known as iXBRL.

A complete corporation tax return UK submission includes three things: the CT600 form itself, your statutory accounts, and your tax computations, all formatted in iXBRL. Most accounting software handles this formatting automatically, which is why choosing the right software matters more in 2026 than ever before.

Who Needs to File a CT600 with HMRC?

This catches more business owners off guard than you might expect. The obligation to file a corporation tax return in the UK is broader than most people assume.

The following must all file the HMRC CT600 form:

• All UK-registered limited companies — whether profitable, loss-making, or inactive
• Non-UK companies that have a branch or permanent establishment in the UK.
• Community Interest Companies (CICs)
• Charities and nonprofit organisations that trade or receive investment income

It is mandatory to file a CT600 whether your company has made a profit or not, made a loss, or had no income at all during the period. Even if HMRC has not yet issued you a formal notice to deliver a company tax return, you are still responsible for registering for corporation tax and filing on time. Waiting for a reminder is not a valid reason for a late submission.

CT600 for Dormant Companies

A dormant company, one that has never traded or has no significant accounting transactions, may not need to file a full CT600. However, there is an important condition: you must formally notify HMRC of your dormant status, after which they will confirm whether a return is required.

The moment any trading activity, income, or significant transaction occurs, your filing obligations begin immediately. Many founders assume a dormant company means zero paperwork, but if HMRC has not been formally notified, penalties can still apply for a missing return.

In case your company is dormant, you are still required to file annual confirmation statements with Companies House and may need to submit dormant accounts. Always confirm your position with HMRC directly to avoid any unexpected penalties.

CT600 Filing Deadlines and Corporation Tax Payment Dates

The most frequent and expensive mistake startup founders make is confusing two separate deadlines that fall at different points in the year.

• CT600 filing deadline UK: 12 months after the end of your accounting period
• Corporation Tax payment timeline: 9 months and 1 day after your accounting period has ended.

The payment deadline comes before the filing deadline, which means you must calculate and pay your tax liability before you have even submitted the return. Many founders miss this and pay interest as a result.

One important note for new companies: a corporation tax accounting period cannot exceed 12 months. If your first accounting period runs longer — which is common for newly incorporated startups — HMRC requires two separate CT600 returns, even though you only file one set of accounts at Companies House. Many first-year founders miss this entirely and receive a penalty notice they never saw coming.

UK Corporation Tax Rates 2026

Understanding which rate applies to your company is essential before completing your HMRC CT600 form, as it directly determines how much you owe.

For most early-stage startups with modest profits, the 19% small profits rate applies. However, if your startup has associated companies — businesses connected through shared ownership or control — the profit thresholds are divided between those entities. The £50,000 lower limit and £250,000 upper limit are each divided by the number of associated companies plus one. This can push you into a higher rate band faster than you expect, so always check your position carefully before filing.

For larger companies with profits exceeding £1.5 million, corporation tax must be paid in quarterly instalments—a different process worth being aware of as your startup scales.

What Documents Are Needed to File a CT600?

Before you begin your CT600 return UK submission, make sure everything is in order. A complete filing requires the following:

Mis-tagged accounts cause immediate rejection by HMRC — line items must align exactly, and using an outdated iXBRL taxonomy leads to non-compliance. Always use up-to-date software that supports the current taxonomy to avoid your submission being rejected on technical grounds.

How to File CT600 After April 2026? — The HMRC CATO Closure

This is the most significant change affecting CT600 filing in the UK right now — and it affects every company regardless of size or complexity.

From 1 April 2026, HMRC’s free CT600 filing service — known as CATO (Company Accounts and Tax Online) — will no longer exist. Filing corporation tax returns remains mandatory, but the free government tool is gone.

What this means for your business:

• Every company must now use commercial software or a professional accountant to file the CT600 online
• There is no longer a free HMRC tool for corporation tax filing
• Some software providers do offer free filing for micro-entity companies—worth researching if you are in the early stages
• For startups with more complex affairs, paid software or professional support will be required

If you do not already use an accounting platform, start exploring your options now. Waiting until your deadline is approaching — particularly if it falls on or after 1 April 2026 — risks disruption to your filing and potential penalties.

CT600 Filing for Foreign Companies with UK Presence

If your business is based overseas but operates in the UK in any meaningful way, your corporation tax obligations may be more significant than you realise.

Non-UK companies with a UK branch or permanent establishment must file the HMRC CT600 form and pay Corporation Tax on profits attributable to their UK activities. A permanent establishment is broadly defined—it can include a fixed place of business, a UK office, or a dependent agent regularly concluding contracts on your behalf in the UK.

If you are keeping your main company abroad but operating in the UK, you need a clean separation of UK versus non-UK activity and thorough bookkeeping to manage your obligations correctly.

From 6 April 2020, non-UK resident companies receiving income from UK property are also subject to Corporation Tax—not Income Tax—on those profits and must submit a CT600 accordingly. Double taxation treaty relief may be available depending on your home country’s treaty position with the UK, which can reduce the risk of being taxed twice on the same income.

Tax Reliefs and Allowances to Claim on a CT600

Your CT600 is where you claim reliefs that reduce your corporation tax bill. Leaving any of these unclaimed means overpaying tax, and HMRC will not remind you to claim them.

R&D Tax Credits: If your startup is developing new technology, software, processes, or products, you may qualify for R&D relief. SMEs may claim an increased deduction of 86% on qualifying R&D expenses, and loss-making SMEs may claim a cash credit of 10% of their surrenderable loss. It is among the most useful reliefs to early-stage companies and can be claimed using the CT600L supplementary page.

Annual Investment Allowance (AIA): The AIA offers a 100% first-year deduction on qualifying plant and equipment up to the value of £1,000,000 per annum. This is because you can offset the cost of qualified equipment, computers and machinery from your income in the year of purchase instead of depreciating it over a few years.

40% First-Year Allowance: In Budget 2025, the government will introduce a new 40% first-year allowance on the qualifying plant or machinery expenditure incurred on or after 1 January 2026. This is an additional relief worth factoring into your tax planning if your startup has invested in qualifying assets.

Loss Relief: If your company made a loss, do not ignore it. Trading losses can be carried forward indefinitely to offset future profits or, in certain circumstances, carried back against prior-year profits to generate a tax repayment. All of this is recorded on your CT600.

Common CT600 Filing Mistakes and HMRC Penalties

Even well-organised companies make avoidable errors when it comes to how to file the CT600. Here are the most common mistakes and exactly what they cost:

Confusing the filing and payment deadlines: Corporation tax is due nine months and one day after your period ends, well before the CT600 filing deadline UK. Paying late means interest charges on top of your bill.

First-year accounting period errors: If you have a first-year accounting period longer than 12 months, HMRC will need two forms of CT600 returns. Many startup founders do not realise this until a penalty notice arrives.

Missing R&D or capital allowance claims: HMRC will not flag reliefs you have missed. They must be actively claimed on your CT600—otherwise you simply overpay.

iXBRL tagging errors: Mis-tagged accounts are rejected automatically. Always use software with a current, supported iXBRL taxonomy.

CT600 penalties HMRC applies are automatic and escalate quickly:

In addition to fines, HMRC imposes daily interest on the late corporation tax payments – at present, it is 7.75% as of June 2025; this is subject to change. In addition to the financial expense, a history of late CT600 filings can result in compliance inspections and damage the reputation of your company with banks and investors at exactly the moment you need their confidence most.

Conclusion

CT600 filing in the UK rewards preparation and punishes delay. The rules are clear, the deadlines are fixed, and the penalties can add up quickly, but with the right approach, the process is far more manageable than it seems.

Stay on top of your deadlines, choose the right software before April 2026, and make sure you claim every relief you are entitled to. While self-filing may work for straightforward companies, businesses with R&D activity, international operations, or complex structures often benefit significantly from professional guidance.

Whether you are filing your first CT600 return in the UK or managing more complex tax obligations, expert support can save you time, reduce risk, and improve accuracy. With Ondemand International, you get end-to-end assistance from company setup to CT600 filing and ongoing compliance, so you can focus on growing your business with confidence.

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