From 1 April 2011 you must file your Company Tax Return online, including supporting documentation, for any accounting period ending after 31 March 2010. Most small companies with simple tax affairs can use the free HMRC Corporation Tax Online filing software but there are also HMRC-tested commercial software packages available.  You must make sure that you or your accountant is able to file online using the required iXBRL format software.  From the same date you must also pay your Corporation Tax electronically for any accounting period.  The HMRC website has further guidance at www.hmrc.gov.uk/ct/managing/company-tax-return/how-to.htm#4