CT600 Filing Guide: Essential Steps for UK Limited Companies in 2026
Navigate CT600 filing seamlessly with this comprehensive guide, covering deadlines, preparation, software changes post-March 2026, and common pitfalls to ensure HMRC compliance.[1][2][5]
What is CT600 and Why Does It Matter?
The CT600 is the core form of the Company Tax Return submitted to HMRC for Corporation Tax, required for all UK limited companies regardless of profit or loss.[2][3] It must be accompanied by statutory accounts and tax computations in iXBRL format, forming a complete digital submission.[1][2] Accurate filing avoids penalties and ensures your business stays compliant—essential reading alongside our in-depth Corporation Tax Guide 2026 for UK Limited Companies.
Key Deadlines You Can't Ignore
Corporation Tax obligations follow strict timelines from your accounting period end:
| Requirement | Deadline |
|---|---|
| Pay Corporation Tax | 9 months + 1 day [4] |
| File CT600 Return | 12 months [1][2][4][5] |
| Submit Company Accounts | 12 months (to Companies House) [4][5] |
Late filing incurs automatic penalties, even for loss-making companies.[1][5]
Step-by-Step Guide to Filing CT600
Follow these proven steps for a smooth process, adapted for changes after 31 March 2026 when HMRC's free online service closes.[1][5][6][7]
Step 1: Prepare Statutory Accounts
Compile full accounts including:
- Profit and loss statement
- Balance sheet
- Notes to the accounts
- Director’s report
These form the CT600 foundation; figures must reconcile with tax computations.[1]
Step 2: Adjust Profits for Tax
Accounting profits differ from taxable profits. Key adjustments include:
- Add back disallowable expenses (e.g., client entertainment)
- Deduct capital allowances
- Apply loss reliefs, group relief, or R&D tax credits
Example: Accounting profit £500,000 + £15,000 disallowables – £25,000 allowances – £50,000 reliefs = £440,000 taxable profit. At 25% rate, liability = £110,000.[1]
Step 3: Register for HMRC Services
Use your company's Unique Taxpayer Reference (UTR) to register for Corporation Tax online services—mandatory for electronic filing.[1][2]
Step 4: Select Filing Software (Critical Post-2026)
- HMRC's free service ends 31 March 2026; no more direct uploads via CATO.[5][6][7]
- Switch to HMRC-approved commercial software (e.g., integrated platforms like Commenda) for iXBRL tagging and submission.[1][2][5]
- Paper filing is rare (e.g., charities, exemptions only—apply via WT1 form).[5][6]
- Software handles CT600, accounts, and computations in one secure package.[7]
Step 5: Complete the CT600 Form
Fill key sections via software:
- Company details and UTR
- Taxable profits and adjustments
- Supplementary pages (e.g., CT600A for loans, CT600B for CFCs)[3]
- Any reliefs or credits
Step 6: Submit and Pay
- Electronically submit via software—mandatory for most.[1][2]
- Pay via BACS, CHAPS, Direct Debit, or bank transfer.[1]
- Retain acknowledgements for records.[1]
Major 2026 Changes: What UK Businesses Need to Know
From 1 April 2026:
- No free HMRC portal; all via approved software (free options exist for simple filings).[5][7]
- iXBRL remains required; good software automates this.[5]
- Deadlines and CT600 form unchanged.[5]
- Companies House shifts follow in 2027 (no abridged accounts).[5]
Prepare now—self-filers face the biggest impact, while accountants typically use software already.[7]
Common Mistakes and How to Avoid Them
- Mismatched figures: Ensure accounts reconcile with CT600.[1]
- Missing iXBRL: Mandatory for digital acceptance.[2][5]
- Wrong deadlines: Even losses require filing.[2]
- No registration: Delays UTR access.[1]
- Overlooking supplements (e.g., box 485 for CT600A).[3]
Recommended Tools and Next Steps
Opt for user-friendly platforms that streamline compliance beyond basic HMRC tools.[1] For personalised support, Cheshire Business Accountants can handle your CT600, saving time and reducing errors.
Tags: CT600, Corporation Tax, HMRC filing, UK limited companies, tax deadlines, iXBRL, 2026 changes
Category: Tax Compliance
Sources
- https://www.commenda.io/compliance/ct600-corporation-tax
- https://sleek.com/uk/resources/corporation-tax-filing/
- https://www.gov.uk/guidance/the-company-tax-return-guide
- https://myiva.co/blog/how-to-calculate-corporation-tax-in-uk/
- https://tinytax.co.uk/guides/ct600-filing-after-march-2026
- https://www.gov.uk/file-your-company-accounts-and-tax-return
- https://anna.money/blog/guides/hmrc-tax-filing-service-closure-how-it-affects-corporation-tax-payers/
- https://help-iris.co.uk/IRIS/ODL/Business_Tax/QG/Internet_Filing/CT600_E-filing_Guide.htm
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