HMRC Company Tax Returns (CT600) – All You Need to Know (2026 Guide)
The CT600 Company Tax Return is a mandatory filing for all UK companies, and getting it right is crucial for avoiding penalties, ensuring compliance, and maintaining clean financial records. This guide provides everything CFOs, finance managers, accountants, and SMEs need to know about CT600 filing, deadlines, computations, and digital submission requirements for 2026.
Use this comprehensive resource as your central pillar for all CT600-related information.
Introduction to CT600
What Is a CT600?
A CT600 is the form that companies submit to HMRC to declare:
Corporation Tax liability
Profit or loss for corporation tax
Capital allowances
Deductions, reliefs, and adjustments
Chargeable gains
Any additional corporation tax-related disclosures
A CT600 must be filed every accounting period, even if:
The company made no profit
No corporation tax is due
The company is dormant (with specific filing rules)
Who Must File a CT600?
You must file a CT600 if you are:
A UK limited company (Ltd)
A foreign company with a UK branch/permanent establishment
An LLP with corporation-tax obligations
A charity with taxable income or gains
A company with chargeable gains, even when dormant
Related guides:
Link: Is Your Company Dormant?
What Does a CT600 Include?
A CT600 form captures:
Corporate financial results
Taxable profit
Corporation tax calculation
Loss relief claims
Adjustments and allowances
Relevant schedules based on your company type
CT600 Accounting Periods
How Accounting Periods Impact Your CT600
A CT600 filing is based on your company’s accounting period, which often matches your Companies House financial year but HMRC rules can create exceptions.
First-Year Filing Scenarios
The first accounting year can lead to multiple CT600 filings.
Scenario 1: Mid-Month Business Start
If you start trading on 15 January 2026:
First return: 15–31 January (17 days)
Second return: 1 February–31 January (12 months)
Scenario 2: April Start Date
If your business starts on 1 April 2026, your CT600 is due 12 months after the period end.
Related guide:
Link: Micro vs Small – Determine Your Company Size
What You Need Before Filing
Financial Information Required
Prepare details including:
Trading income
Allowable expenses
Capital allowances
Loan relationships
Adjustments and disallowable
Trading vs non-trading income
Tax Computations
Your computations explain how you arrived at your taxable profit. These must align perfectly with your CT600 entries.
Related guide:
Government Gateway Access
You will need:
Government Gateway user ID
Password
Company UTR (Unique Taxpayer Reference)
iXBRL-Tagged Accounts and Computations
HMRC requires:
iXBRL-tagged final accounts
iXBRL tax computations
Use of correct FRC taxonomy
Why iXBRL Is Mandatory
iXBRL ensures structured, machine-readable reporting for accurate HMRC validation.
Related guides:
Link: What Is iXBRL?
CT600 Form Breakdown
Mandatory Sections
The CT600 includes:
Company information
Accounting period
Turnover
Capital allowances
Deductions and reliefs
Loan relationships
Chargeable gains
Tax calculation
Optional Sections Depending on Company Type
Some sections apply only to:
Charities
Property companies
Investment companies
Group structures
How Errors Lead to HMRC Rejection
Common problems include:
Incorrect periods
Missing schedules
Misaligned computations
Wrong iXBRL taxonomy
Related guide:
How to File Your CT600 (Step-by-Step)
Step 1: Prepare Final Accounts
Must be fully iXBRL-tagged.
Step 2: Prepare Tax Computations
Include all adjustments, allowances, and profit calculations.
Step 3: Complete the CT600
Using HMRC’s software or approved external software.
Step 4: Submit Online to HMRC
You will receive an acknowledgment upon successful submission.
Step 5: Keep Records for 6 Years
Maintain:
Accounts
Computations
Proof of submission
Related guides:
CT600 Filing Deadlines
CT600 Deadline
Your CT600 is due 12 months after the end of your accounting period.
Examples:
Year end 31 Dec 2025 → Deadline 31 Dec 2026
Year end 31 Mar 2026 → Deadline 31 Mar 2027
Corporation Tax Payment Deadline
Tax must be paid 9 months and 1 day after period end.
Why Deadlines Differ
HMRC requires tax paid earlier so they can recover liabilities before returns are finalised.
CT600 Penalties (2026 Rules)
Penalties for Late Filing
1 day late → £100
90 days late → + £100
6 months late → 10% of owed tax
12 months late → further 10%
Penalties for Consecutive Late Filings
If late 3 years in a row, the £100 penalties increase to £500 each.
Penalties for Incorrect Filings
Incorrect CT600s may trigger:
Amendments
Compliance checks
Additional interest
Related guides:
Common CT600 Mistakes
Incorrect Accounting Periods
This is the #1 cause of rejection.
Incorrect or Missing iXBRL Tags
Mis-tagged accounts → immediate rejection.
Computation Mismatches
Line items must align exactly.
Wrong Taxonomy Version
Using an outdated taxonomy leads to non-compliance.
Missing Attachments
All schedules and accounts must be included.
Related guides:
CT600 for Special Company Types
Micro Entities
Micro entities often file:
FRS 105 accounts
CATO submissions
Simplified CT600 schedules
FRS 105
Micro-entity accounting standard.
CATO
Simplified filing route for eligible micro entities (shutting down in 2026).
Related guides:
Link: Micro vs Small
Charities
Charities file CT600 when they have:
Taxable trading income
Chargeable gains
Non-charitable activities
Related guide:
Overseas Companies
Foreign companies with UK activities must file.
Related guides:
Dormant Companies
Dormant companies file only under specific conditions.
Digital Filing Requirements (iXBRL)
What Needs to Be Tagged
Accounts
Computations
Notes
Disclosures
Using Correct FRC Taxonomy
Mandatory for HMRC acceptance.
Why iXBRL Reduces HMRC Rejection
Structured data improves validation accuracy.
Validation & Pre-Checks
Pre-submission reviews significantly reduce error rates.
Related guides:
How DataTracks Helps
iXBRL Tagging
We prepare fully FRC-compliant iXBRL files.
Error Reduction
Automated validation detects issues before submission.
CT600 Support
We assist in preparing filing-ready accounts.
Submission-Ready Output
Ensuring accurate, complete and timely filings.
For expert CT600 and iXBRL support:
Email: enquiry@datatracks.co.uk
