A London Assembly member has questioned why the Metropolitan Police are still using Windows XP on tens of thousands of computers.
Conservative Andrew Boff told the BBC it would not be cost-effective to keep paying for patches to make sure the operating system was secure.
Last year, a Freedom of Information request by the tech site Motherboard found 35,000 Met PCs ran XP.
Mr Boff said this had fallen to 27,000, which remained “worrying”.
“I have fond memories of XP, I’ve only just got rid of it myself a few months ago,” said Mr Boff.
Microsoft ceased providing free security and technical support to XP users in April 2014.
Currently, the Met is carrying out a plan to upgrade thousands of its computers to Windows 8.1.
Mr Boff said the force should instead upgrade to the latest Windows operating system, Windows 10.
“I’ll be asking a question and, depending on the written reply I get, I’ll then be questioning the mayor on whether or not they should start a review,” he said.
“We spend an awful lot of money on information technology – we’ve got to get the best bang for our buck.”
£1.65m extended support
The Met Police confirmed that, by the end of September, a further 6,000 desktops will be upgraded to Windows 8.1, reducing the overall number of PCs still running XP to 21,000.
It added that those computers still running the older operating system will be covered by an extended support arrangement with Microsoft until April 2017 – at a cost of £1.65m.
The Met said a large amount of “legacy software” meant that the upgrade plan was not as easy as it might be at many other organisations.
“Further plans are being developed to address the remaining XP desktops,” it added, “including reducing the overall number used by the organisation, replacing with laptops, tablets and disposing of equipment that cannot support Windows 8.1 and beyond.”