The UK government has agreed to pay Microsoft more than £5m to extend support for Windows XP by one year.

Tens of thousands of PCs used by officials in central and local government are still running Windows XP, despite support for the OS ending on April 8.

The deal will see Microsoft provide critical and important security updates for XP, Office 2003 and Exchange 2003 to UK public sector organisations until April 8, 2015.

The extension will cost £5.584m, a sum the government claims is some £20m less than the "standard pricing" for extended support charged by Microsoft.

When asked why multiple government organisations have failed to migrate from XP, despite the end of support date being widely advertised, a spokesman for the UK Cabinet Office said such a transition can be "costly and complex" and "the main priority for public sector organisations will be to ensure a seamless transition".

The government expects "the majority of organisations to be transitioned off XP by April 2015", according to the spokesman.

"This is an important deal, which will provide continuity for all eligible government and public sector organisations while they migrate on to alternative operating systems," according to a Cabinet Office statement.

Public sector organisations must sign up for extended support, rather than automatically being eligible, and to qualify a government body must be able to demonstrate it has a plan in place to migrate from XP.

According to information released following FOI requests by The Register, tens of thousands of public sector computers will miss the April 8 date to move from XP, with many of these machines inside HM Revenue and Customs and the NHS.