Guide

File your Company Tax Return (CT600)

How to file your Company Tax Return (CT600) including deadlines, payment requirements, iXBRL tagging, and quarterly instalment rules for large companies. Critical: payment deadline is 9 months, filing deadline is 12 months.

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What is a Company Tax Return (CT600)

Every limited company and public limited company (PLC) must file a Company Tax Return with HMRC for each accounting period. This is separate from your annual accounts filed with Companies House.

The CT600 form tells HMRC how much Corporation Tax your company owes. Even if you made no profit or your company is dormant, you must still file a return.

Critical deadlines: Payment before filing

Important: Your Corporation Tax payment deadline is earlier than your filing deadline. Many businesses miss this and face penalties.

WARNING

⚠️ Payment deadline is BEFORE filing deadline

Critical timing difference:

  • Payment due: 9 months and 1 day after accounting period ends
  • Return filing due: 12 months after accounting period ends

Example: If your accounting period ends 31 December 2024:

  • Pay Corporation Tax by: 1 October 2025 (9 months 1 day)
  • File CT600 by: 31 December 2025 (12 months)

You must estimate and pay your tax 3 months before you file the return. Late payment triggers automatic interest charges (Bank of England base rate + 4%, currently 8% as of December 2025).

This timing creates a cash flow challenge: you must pay before finalizing accounts. Plan ahead:

  • Estimate your Corporation Tax liability during the year
  • Set aside funds monthly (profit × 19% or 25% depending on profit level)
  • Use accounting software to track profit in real-time
  • Consider quarterly management accounts to avoid surprises
Corporation Tax payment and filing deadlines

What you must include in your CT600

Your Company Tax Return must include:

  • CT600 form: Corporation Tax calculation and company details
  • Full statutory accounts: Balance sheet, profit and loss, notes - all in iXBRL format
  • Corporation Tax computation: Detailed calculation showing how you arrived at taxable profit - also in iXBRL
  • Supporting schedules: Capital allowances, losses carried forward, R&D claims if applicable

All documents must be tagged using iXBRL (inline eXtensible Business Reporting Language) - this is mandatory and has been since 2011.

How to file your CT600 online

You must file your Company Tax Return online. HMRC does not accept paper returns except in exceptional circumstances.

  1. Get your Corporation Tax UTR and activation code

    HMRC sends your Unique Taxpayer Reference (UTR) when you register for Corporation Tax. You need this plus an activation code to file online.

  2. Prepare accounts in iXBRL format

    Use HMRC-compatible software or your accountant to create iXBRL-tagged accounts and tax computation. Manual tagging is technically possible but extremely complex.

  3. Complete CT600 form online

    Log into HMRC's Corporation Tax online service or use commercial software. Enter company details, accounting period, and tax calculation.

  4. Upload iXBRL accounts and computation

    Attach your tagged accounts and tax computation files. HMRC's system validates the tags automatically.

  5. Submit before deadline

    File at least 48 hours before deadline to allow for technical issues. Keep confirmation email as proof of filing.

LIMITED COMPANY Requirement

All limited companies must file CT600 - even dormant companies

Dormant companies (no significant accounting transactions in the period) must still file a Company Tax Return, though the process is simpler:

  • Use the shortened CT600 (Version 3) for dormant companies
  • No Corporation Tax to pay (£0 liability)
  • Still need iXBRL-tagged dormant accounts
  • Same 12-month filing deadline applies

Newly incorporated companies: Your first CT600 covers from incorporation date to your first accounting period end (usually 12 months, but can be up to 18 months for first period).

Public Limited Companies (PLCs): Same CT600 requirements as private limited companies, but typically face additional scrutiny and may need more detailed disclosures.

Comparison to other structures:

Only entities that are NOT companies (sole traders, partnerships, LLPs) do not file CT600. They file Self Assessment tax returns instead.
Dormant companies and Corporation Tax

Large companies: Quarterly instalment payments

If your company's profits exceed £1.5 million, you cannot wait 9 months to pay. You must pay Corporation Tax in quarterly instalments throughout and after the accounting period.

This is one of the most commonly overlooked requirements when companies grow past £1.5m profit for the first time. See our Corporation Tax basics guide for full details on quarterly instalment schedules, payment dates, and strategic planning considerations.

Penalties for late filing

HMRC penalties for late CT600 filing are automatic and escalate quickly. Even if you have no tax to pay (dormant company), late filing penalties still apply.

Interest on late payment

Separate from filing penalties, HMRC charges interest if you pay your Corporation Tax late. This applies even if you file the CT600 on time but pay late.

iXBRL tagging requirements

Since 2011, all companies must submit accounts and tax computations using iXBRL (inline eXtensible Business Reporting Language). This allows HMRC to process returns automatically.

Submitting untagged documents will cause your return to be rejected.

Common mistakes to avoid

  • Missing the payment deadline: Remember 9 months 1 day for payment, 12 months for filing - payment comes first
  • Forgetting iXBRL tagging: Untagged returns are rejected - use compatible software or hire an accountant
  • Not checking for quarterly instalments: Crossing £1.5m profits triggers quarterly payments - easy to miss if you've never been 'large' before
  • Ignoring associated companies: Multiple companies under common control share the £1.5m threshold
  • Assuming dormant = no filing: Dormant companies must still file CT600 (simplified version)
  • Using old CT600 versions: HMRC updates the CT600 form annually - use the correct version for your accounting period
  • Not keeping evidence: Save your filing confirmation email and payment references - HMRC can query returns up to 4 years later (6 years if suspected errors)

Get help with filing

Most companies use an accountant to file CT600 returns due to the complexity of:

  • Corporation Tax computations (adjusting accounting profit to taxable profit)
  • iXBRL tagging requirements
  • Capital allowances and tax relief claims
  • Quarterly instalment calculations for large companies

If you file yourself, use HMRC-approved commercial software that handles iXBRL tagging automatically. Manual tagging is extremely error-prone.