HMRC Company Tax Returns (CT600) – All you need to know!

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: UK Tax for Overseas Companies

• Link: Is Your Company Dormant?

• Link: Charities and Corporation Tax

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:

• Link: Comprehensive Guide to UK Corporation Tax Filing

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: HMRC-Approved Taxonomies Explained

• 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:

• Link: How to Reduce HMRC Rejection

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:

• Link: Choosing Between Paper and Online Returns

• Link: Filing on Time Post Lockdown

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:

• Link: Missed Your Corporation Tax Deadline?

• Link: Impact of Missing HMRC Tax Return Deadline

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:

• Link: HMRC’s Experience with iXBRL Data

• Link: iXBRL Submission Rejections – A Study

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 Entities – HMRC Definitions

• Link: Micro vs Small

Charities

Charities file CT600 when they have:

• Taxable trading income

• Chargeable gains

• Non-charitable activities

Related guide:

• Link: Charities and Corporation Tax

Overseas Companies

Foreign companies with UK activities must file.

Related guides:

• Link: UK Tax for Overseas Companies

• Link: Registering a Non-Resident Company

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:

• Link: What You Need to Know About iXBRL Tagging

• Link: HMRC-Approved Taxonomies Explained

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