A CT600 is the corporation tax return that UK limited companies, foreign companies with UK branches, and clubs or associations file with HMRC. It is filed online using commercial software within 12 months of the accounting period end, while the tax itself is due earlier, 9 months and 1 day after the period ends.
CT600 was supposed to be done by accounting practices without much hindrance, yet many are facing challenges in completing this compliance task effectively, leading to last-minute rushes and late-night checks.
Corporation tax is at its peak of 25% for profits above £250,000, increased scrutiny around R&D claims, and tighter filing enforcement by HMRC have made it even more important to get the CT600 filing right at the first place.
Let’s break down the CT600 properly and make it manageable.
Key Takeaways
- A CT600 must be filed online using commercial software within 12 months of the accounting period end.
- Corporation tax is due 9 months and 1 day after the period ends; companies with profits over £1.5m pay in quarterly instalments.
- For the financial year from 1 April 2026: 19% up to £50,000, 25% above £250,000, marginal relief in between.
- The joint HMRC and Companies House filing service (CATO) closed on 31 March 2026, so commercial software is now essential.
- Late filing costs £100 at once and £200 after 3 months, plus 10% tax-geared penalties at 6 and 12 months.
What Is CT600?
A CT600 is the corporation tax return form that limited companies in the UK, foreign companies with UK branches, and clubs/associations must file with HMRC annually.
It tells HMRC:
- Company taxable profits
- Corporation tax owed
- Reliefs and allowances claimed
- Loans to participators
- R&D tax credits (where applicable)
This form must be filed electronically using approved software within 12 months of the accounting period. Non-compliance will have legal consequences.
Which of Your Clients Must File a CT600 Form?
All limited companies and organisations in the UK registered with Companies House are required to file a CT600 form. This includes limited companies, foreign companies with UK branches, and clubs or co-operatives. Your clients are still liable to fill in the CT600 form even if they make a loss or have no Corporation Tax to pay.
What Does a CT600 Form Contain? A Section-by-Section Guide
A CT600 form is the UK corporation tax return submitted to HMRC, containing your client company’s financial details for an accounting period to calculate tax owed.
Here’s what this form contains:
- Company Details: Under it comes the registered name, UTR (Unique Taxpayer Reference), and the specific accounting period covered.
- Income and Profits: In this section, all your client company’s turnover, trading profits or losses, and other income like bank interest are added.
- Tax Calculation: Total profit, deductions, reliefs (e.g., R&D), and the final Corporation Tax due are included.
- Capital Allowances: Adjustments for equipment purchases and assets.
- Supplementary Pages: Used for specific circumstances such as charitable, exemptions, or foreign income.
When Is the CT600 Deadline and When Is Corporation Tax Due?
The CT600 is due 12 months after the accounting period ends, and corporation tax is due earlier, 9 months and 1 day after it ends.
Deadlines associated with corporation tax have often confused your clients. Here’s something that you will need to remember:
- The CT600 filing deadline is 12 months after the accounting period end
- The corporation tax payment deadline is 9 months and 1 day after the accounting period end
For example, if your clients’ accounting year ends on 31 October 2025, the CT payment is due by 1 August 2026, while the CT600 filing deadline is 31 October 2026. Ideally, it is important for you and your client to complete the filing and payment before August.
Missing the payment deadline will lead to the application of interest, and missing the filing deadline will lead to penalties immediately.
Large companies pay differently. A company with profits over £1.5 million (limit divided by the number of associated companies plus one) pays in quarterly instalments, and a company over £20 million pays earlier (GOV.UK).
| Company size | Profits | When corporation tax is paid (12-month period) |
| Standard | Up to £1.5m | One payment, 9 months and 1 day after the period ends |
| Large | £1.5m to £20m | Four instalments, in months 7, 10, 13 and 16 from the period start |
| Very large | Over £20m | Four instalments, in months 3, 6, 9 and 12 from the period start |
What Are the Penalties for Filing a CT600 Late?
A late CT600 costs £100 immediately and a further £100 after 3 months, followed by tax-geared penalties of 10% of unpaid tax at 6 and 12 months.
These days, HMRC has been very strict when it comes to enforcing corporation tax compliance. To maintain and improve compliance, HMRC has been using the penalty tool very effectively.
Penalties for late CT600 filings are:
- £100 immediately if late
- Another £100 if more than 3 months late
- 10% of unpaid tax if 6 months late
- An additional 10% if 12 months late
Repeat offenders pay more: where three returns in a row are late, the flat penalties rise to £500 each.
As an accounting practice handling the corporation tax filings for your clients, it is your responsibility to ensure the deadlines for filing and payment of corporation tax are met. Otherwise, it will negatively impact your clients and your reputation.
How Do You File a CT600? Step-by-Step Process for UK Accountants
To file a CT600, gather the records, confirm the accounting period, work out taxable profits, complete the return, file it through commercial software, and pay by the due date.
Here’s a clean workflow that your practice must follow while filing CT600 for your clients.
Gather All the Records
Gather and prepare all the necessary records from your client, such as company accounts, profit and loss, and details of relief and allowances.
Confirm Accounting Period
Confirm the start and end dates of your clients’ corporation tax period, and also make your client aware of it.
Work Out Taxable Profits
Begin by calculating your client’s profits and then work on the allowances and reliefs.
Complete CT600
Once the calculations are done, begin filling the form by entering your clients’ company details, financial results, tax breaks, and corporation tax calculations.
File Online
Submit the CT600 form through approved commercial software. HMRC’s free joint filing service closed on 31 March 2026.
Pay Corporation Tax
Once the submission of the form is done, ensure your client settles the bill within nine months and one day, which is the deadline for corporation tax payment. Large companies pay by instalments instead.
Keep Records
Maintain the record of the CT600 and supporting documents after the end of the accounting period, as it will come in handy for audit and future references.
Which Is the Best CT600 Software for UK Accounting Practices in 2026?
No single product suits every practice. IRIS, TaxCalc, BTCSoftware, CCH and Digita are commonly used, so choose by client volume, integration and workflow automation needs.
HMRC has made it compulsory to use approved software for digital filing. The joint HMRC and Companies House online filing service (CATO) closed on 31 March 2026, so every practice now files through commercial software (Xero guide). This makes software selection more critical than ever for your practice.
Some of the popular platforms that you must try out are:
- IRIS
- TaxCalc
- BTCSoftware
- CCH
- Digita
Select the software after considering:
- Volume of clients you handle
- Integration with other accounts software
- Requirement of workflow automation needs
But equal attention must also be placed on the process of preparing them. Most accounting practices are unable to get that balance due to multiple factors like volume, complexity and so on. They have turned towards corporation tax outsourcing services for assistance and to get that balance. It’s time for you to make that turn.
What Are the Most Common CT600 Errors Accountants Make?
The most common CT600 errors are incorrect marginal relief, R&D misclassification, accounts that do not reconcile to the tax computation, the wrong accounting period, and late filing under capacity pressure.
There is a possibility that even your experienced accounting team might commit mistakes in CT600 filings. Most of these mistakes happen due to the rise in complexities, short deadlines, and high volumes, leading to pressure. When pressure rises, errors occur.
Let’s break down the most common mistakes you will face and how to prevent them.
Incorrect Marginal Relief Calculation
Marginal relief is the reduction that stops tax jumping from 19% to 25% when profits fall between £50,000 and £250,000. It equals (£250,000 minus profits) × 3/200, deducted from tax at 25% (HMRC CTM03925). These rates apply to the financial year from 1 April 2026 (GOV.UK rates).
Since the reintroduction of the 25% corporation tax main rate, marginal relief calculations have become more complex.
Accountants must consider:
- Small profit rate (19%)
- Main rate (25%)
- Marginal relief band (£50,000–£250,000)
- Associated companies affecting thresholds
A small negligence in oversight, such as failing to adjust thresholds for associated companies, will lead to wrong tax liability.
How to avoid it:
- Always confirm with your client about their associated company status
- Use updated tax software reflecting current corporation tax rates
- Build a review checkpoint for marginal relief cases
R&D Claim Misclassification
R&D claims have become a major compliance hurdle with HMRC increasing its scrutiny due to its misuse and inaccurate submissions in recent years.
Common issues include:
- Claiming routine software upgrades as R&D
- Incorrect SME vs RDEC classification
- Inadequate technical narrative documentation
- Overstated qualifying expenditure
Incorrect R&D reporting on the form can cause enquiries, repayment demands, or penalties.
How to avoid it:
- Validate eligibility criteria carefully
- Ensure technical justifications are documented
- Separate qualifying and non-qualifying costs clearly
- Use specialist review for high-value claims
R&D claims are important for reducing your client’s corporation tax, but they must be defensible.
Mismatch Between Accounts and Tax Computation
It is one of the common and avoidable errors where the profit per statutory accounts does not reconcile with the starting point in the tax computation. HMRC might flag the inconsistencies.
Causes for this mismatch could be:
- Adjustments missed in the tax comp
- Depreciation was not properly added back
- Capital allowances misaligned
- Dividends treated incorrectly
Even small mismatches create:
- HMRC queries
- Processing delays
- Client anxiety
How to avoid it:
- Always reconcile profit before tax to taxable profit
- Run a final cross-check before submission
- Use a structured CT600 preparation checklist
Accuracy in reconciliation reduces risk dramatically. Reconciliation tools such as Corby X and error-detection checks such as Sherlock support this review at Corient.
Filing for the Incorrect Accounting Period
When your client shortens their accounting period, extends their year-end, or changes company structure, the problems start. The CT600 must reflect the correct accounting period, and sometimes more than one return may be required.
Errors here can result in:
- Rejected filings
- Duplicate returns
- Incorrect tax calculations
- Payment mismatches
How to avoid it:
- Confirm Companies House filing period
- Confirm the corporation tax period with HMRC
- Document year-end changes clearly
- Split long accounting periods correctly (if exceeding 12 months)
Late Filing Due to Capacity Issues
Many accounting practices these days are suffering from capacity constraints due to accountant shortages in the UK market. On the other hand, the volume of CT600 filings is rising along with other factors like bottlenecks, last-minute client queries, and staff absences, leading to delays in filings.
Late CT600 filing penalties from HMRC begin at £100 and escalate quickly, and repeated late filings increase penalties significantly.
But here’s the bigger risk:
- Client frustration
- Lost trust
- Damaged reputation
How to avoid it:
- Start CT600 preparation earlier in the cycle
- Spread workloads quarterly
- Standardise preparation processes
- Use structured review workflows
Consider expanding your capacity by availing the corporation tax outsourcing services during peak periods.
People Also Ask
What is CT600 and who needs to file it?
A CT600 is the corporation tax return that HMRC requires from UK limited companies, foreign companies with UK branches, and clubs or associations. It reports taxable profits, tax owed, reliefs claimed, loans to participators and R&D claims. It is filed each accounting period electronically using commercial software, even where the company made a loss or owes no tax.
How does the corporation tax rate affect what I put in the CT600?
The rate decides the tax figure in the return. For the financial year starting 1 April 2026, companies pay 19% on profits up to £50,000 and 25% above £250,000. Profits in between are taxed at 25% less marginal relief. Associated companies lower both limits, so confirm each client’s associated company status before calculating.
What are the CT600 filing deadlines for UK companies in 2025/26?
The CT600 is due 12 months after the end of the accounting period. Corporation tax is due earlier, 9 months and 1 day after the period ends. For a 31 October 2025 year end, payment is due 1 August 2026 and filing by 31 October 2026. Large companies with profits over £1.5 million pay by quarterly instalments instead.
What is the difference between CT600 and CT603?
The CT600 is the corporation tax return itself, the form on which a company reports profits and calculates tax. A CT603 is a notice to deliver a company tax return, which HMRC sends to request a CT600. Receiving one means HMRC expects a return, so the practice should check the accounting period and file promptly.
Can a CT600 be longer than 12 months?
No. A corporation tax accounting period cannot be longer than 12 months. If statutory accounts cover a longer period, for example 15 months, the practice must split it into two accounting periods and file two CT600 returns, each with its own computation. Each period also has its own payment deadline, so check both dates carefully.
Do you have to file a CT600 for a dormant company?
Usually not. A dormant company does not need to file a Company Tax Return (CT600) unless HMRC asks for one, although it must still file annual accounts and a confirmation statement with Companies House. If HMRC issues a notice to deliver a return, the company must file it by the date stated.
Conclusion
CT600 does not help your accounting firm to grow; it constrains you. Yes, it is mandatory, but it is repetitive, compliance-heavy, and capacity-draining. As an experienced accounting practice owner, you will need to ask yourself a question: “Should my senior team be preparing CT600 returns or advising clients on growth?”
Enter Corient, an accounting outsourcing service provider serving accounting practices in the UK since 2011. We support accounting practices with structured, compliant, review-ready CT600 preparation through our corporation tax outsourcing services and act as a seamless extension of your team. Our wider tax outsourcing support and year end accounts services keep accounts, computations and returns aligned, with our Coventry-based team backed by AI tools such as Corby X and Sherlock.
Keep your clients satisfied by connecting with us and transforming your CT600 filing process.
