The Metropolitan Police is still running the out-of-date Windows XP operating system on 27,000 computers, new figures from the UK government have revealed.

Microsoft stopped releasing official security updates for XP in April 2014 – however organisations not able to update their systems by this time could pay for additional support. The deal, which was used by the Met, cost the government over £5m but it ended in April last year.

Just prior to support being cut off it was revealed the London police had 35,000 computers still running Windows XP. At the time, through its modernisation programme, the police had reportedly set a deadline of March 2016 to make the jump to Windows 8.1.

Yet new figures, first published by Greater London Authority Conservatives after correspondence with mayor Sadiq Khan, disclosed that only 8,000 computers have been updated so far.

The target, according to the government, is for a further 6,000 PCs by September 2016 to be upgraded – however this of course still leaves 21,000 computers lagging behind.

The mayor said: "The MPS [Metropolitan Police Service] are developing further plans to address the outstanding XP desktops including reducing the overall number and disposing of equipment that cannot support Windows 8.1."

In response to the news, London Assembly Member Andrew Boff criticised the slow pace of upgrades and said that sensitive data held on outdated PCs may now be at risk of compromise. "Windows XP is well past its sell-by date," he claimed. "The Met should have stopped using Windows XP in 2014 when extended support ended, and to hear that 27,000 computers still use it is worrying."

He continued: "My major concern is the security of Londoners' information on this dangerously out-of-date system, but I would also like to know how much money the Met has wasted on bespoke security updates.

"I also question the choice to upgrade to Windows 8.1; this is neither the newest nor the most used version of Windows. Staff are likely to be more familiar with Windows 10, but most importantly it will be supported further into the future."