Limited companies and sole traders across the UK are being reminded by HM Revenue & Customs (HMRC) of impending changes to corporation tax (CT).

The changes mean small businesses including contractors and freelancers need to file all tax returns online after the end of March 2011.

Although this seems like a long time in the future, it will impact all accounting periods after the end of March 2010, the HMRC explained.

Mark Holden, director of HMRC's Carter Programme, said: "While March 2011 might seem a long way away, 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're 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."

CT returns need to be filed over the internet using a specific data format Inline XBRL or iXBRL.