The HMRC software that accepts corporation tax returns filed online has not been updated for the new corporation tax rates that came into effect on 1 April 2011. So companies that are due to pay corporation tax at anything other than the small profits rate (20%) cannot yet file their corporation tax return for a period that ends after 31 March 2011. The next upgrade to the HMRC software is expected in October 2011, after which it should be possible to file all corporation tax returns.

Returns requiring the new CT rate

http://www.ion.icaew.com/TaxFaculty/22155