Guidance: Filing your Company Tax Return online.
HMRC requires companies to file their Corporation Tax returns online using recognised commercial software. This applies to all limited companies, with paper returns only permitted if you have a reasonable excuse or are filing in Welsh. The software can also be used to produce iXBRL accounts and computations, and in some cases, to file annual accounts with Companies House.
Source: https://www.gov.uk/government/publications/corporation-tax-commercial-software-suppliers. Summarised automatically from the official source — general information, not advice.