Corporation tax filing software prepares and submits the CT600 return to HMRC on behalf of a limited company. Every company that is subject to UK corporation tax must file a CT600 within nine months and one day of its accounting period end, accompanied by the company's statutory accounts in iXBRL format. Getting the software right matters: errors in CT600 submissions can trigger HMRC enquiries and lead to penalties.

This guide covers what to look for in corporation tax software, the main platforms available to UK accountants, and how the filing process works.

Understanding the CT600 and iXBRL requirements

The CT600 is the main corporation tax return form. It captures the company's taxable profits, allowances claimed, and the resulting tax liability. For most companies, the CT600 must be accompanied by the statutory accounts (profit and loss account and balance sheet) tagged in iXBRL format. iXBRL (Inline eXtensible Business Reporting Language) is a data format that allows HMRC's systems to read and process financial data electronically.

HMRC requires iXBRL-tagged accounts for almost all companies. There are limited exemptions, primarily for very small dormant companies filing nil returns. In practice, any accounting software that produces statutory accounts for CT600 submission purposes will need to generate iXBRL output.

Key deadlines and penalties

The CT600 must be filed within 12 months of the accounting period end. The corporation tax payment itself is due nine months and one day after the year end for small companies. Large companies (profits above £1.5 million) pay in quarterly instalments.

Late filing penalties start at £100 for returns up to three months late, rising to £200 for returns more than three months late. Persistent late filers face higher flat-rate penalties. If the return is more than 18 months late, a further penalty of 10% of the unpaid tax applies.

What to look for in corporation tax software

Full CT600 coverage

The CT600 has numerous supplementary pages for specific situations: research and development credits, capital allowances, losses, group relief, and more. Ensure the software covers the pages relevant to your client base. If you have clients claiming R&D tax credits, check that the R&D supplementary pages are fully supported.

iXBRL accounts production integration

The most efficient workflow links the accounts production module directly to the corporation tax return. This avoids the need to re-enter financial data and ensures consistency between the accounts and the CT600. Most major practice software vendors offer this integration within their own product suite.

Capital allowances calculation

The Annual Investment Allowance (AIA) and capital allowances pools must be calculated correctly and carried forward year on year. Software should maintain capital allowances pools across periods and handle the transition between different AIA limits automatically.

Group and associated company handling

For practices with clients that are part of corporate groups, group relief allocation and associated company thresholds must be handled correctly. The associated companies rules affect the corporation tax rate applied, so software needs to track relationships between entities.

HMRC filing and payment tracking

The software should file directly to HMRC via the Government Gateway and confirm receipt. It should also track payment due dates and outstanding liabilities to help with client communication and cash flow planning.

The leading corporation tax platforms

TaxCalc Corporation Tax

TaxCalc is one of the most widely used CT platforms in the UK practice market. It integrates with TaxCalc Accounts Production for iXBRL output and covers the full CT600 form set. Capital allowances computation is built in, and the software handles R&D credits, losses, and group company scenarios. Pricing is per module, with an unlimited-client licence available for larger practices.

IRIS Business Tax

IRIS Business Tax is the corporation tax module within the broader IRIS suite. It connects to IRIS Accounts Production and IRIS Practice Management. For firms already using IRIS across their practice, Business Tax is the natural fit for CT work. It covers the full CT600 including supplementary pages and supports iXBRL output.

Digita Business Tax (Thomson Reuters)

Digita Business Tax is used in many larger accountancy firms. It handles complex CT scenarios including group relief, research and development claims, and ring-fence profit computations for specific industries. It integrates with Digita Accounts Production for iXBRL filing. Pricing reflects its positioning in the mid-to-large practice segment.

BTC Software Corporation Tax

BTC Software offers a well-regarded corporation tax module as part of its practice suite. It is competitively priced and handles the standard CT600 form set with iXBRL accounts integration. BTC is popular with smaller and mid-sized practices that want comprehensive coverage without the cost of the larger vendors.

Xero Tax (corporation tax)

Xero Tax supports CT600 filing for limited companies and pulls data directly from a connected Xero ledger. For practices where clients use Xero for their bookkeeping, this reduces duplication. The CT functionality is less extensive than dedicated CT platforms for complex cases, but handles straightforward company returns efficiently.

How the filing process works

For most practices, the workflow for a CT filing runs as follows. First, the accounts are prepared and reviewed in the accounts production module. Once finalised, the accounts are linked to the CT600 computation, with income, expenditure, and adjustments flowing through automatically. The capital allowances computation is reviewed and adjusted where necessary. Deferred tax is calculated. The CT600 is reviewed, signed off by the client, and filed electronically to HMRC alongside the iXBRL-tagged accounts. A copy of the filing confirmation is retained in the client file.

For a broader view of the tools available to UK accounting practices, explore our tax software reviews.

Key takeaways

  • CT600 returns must be filed within 12 months of the accounting period end, with corporation tax due nine months and one day after the year end for small companies.
  • Most CT filings require iXBRL-tagged accounts, which means accounts production and CT software must be integrated or compatible.
  • TaxCalc, IRIS Business Tax, Digita, and BTC Software are the leading platforms for corporation tax in UK practices.
  • Always verify that the software covers the specific CT600 supplementary pages your clients need, particularly R&D, group relief, and capital allowances.
  • Capital allowances pools carry forward between years: choose software that maintains these automatically to avoid computation errors.

Frequently asked questions

What is iXBRL and why is it required for CT600 filing?

iXBRL (Inline eXtensible Business Reporting Language) is a tagging format that allows HMRC's systems to read structured data from company accounts electronically. HMRC requires accounts accompanying the CT600 to be submitted in iXBRL format so that financial data can be processed automatically. Most accounts production software generates iXBRL output as part of the finalisation process.

When does corporation tax have to be paid?

For small companies with profits up to £1.5 million, corporation tax is due nine months and one day after the end of the accounting period. For large companies with profits above £1.5 million, quarterly instalment payments apply throughout the year. The number of associated companies affects where the profit thresholds are set, which is why software must track company relationships correctly.

Can I file a corporation tax return without an accountant?

Yes, companies can file CT600 returns directly using HMRC's online filing service, provided they have the correct software to produce a compliant CT600 and iXBRL-tagged accounts. HMRC's CATO (Company Accounts and Tax Online) service supports this for companies that produce their own accounts. For companies with straightforward finances and a nil or simple liability, this is feasible. For anything more complex, an accountant's involvement is advisable.