All firms will have to file their corporation taxes (CT) online from April 2011.
HM Revenue & Customs is writing to half a million companies and tax agents this monthto warn of the change.
AnHMRC leaflet explains how, after 31 March 2011, CT returns must be filed online in a specified data format (known as inline XBRL or iXBRL).
Companies will be able to file online either through commercially available software or by using HMRC's own CT filing software, which is aimed at companies with less complex tax affairs.
HMRC's software will be available in November, said Mark Holden, director of HMRC's Carter Programme."Agents and businesses need to be thinking about these changes now, as they will affect accounting periods ending after 31 March next year.
"HMRC is hoping to release its CT filing software towards the end of this year. We are also working closely with software developers, and a number of commercial products will be available to support the iXBRL standard from November 2009, with more to follow."