Trusts have been told that they have one week to get Microsoft premier support in place if they want to take advantage of a government deal to extend support for Windows XP.

A letter from the Cabinet Office and the Department of Health says trusts must have an agreement in place before the first full patch release, which is due next Tuesday, 13 May.

Trusts with fewer than 250 devices to support can register for the new custom support at no cost. Trusts with bigger XP estates will either need to register for the custom support or buy a premier support agreement at a reduced rate negotiated by Crown Commercial Services.

Whatever position they are in, the letter warns: “It is imperative that your organisation clearly understands the risk that is being placed on it, should the decision be not to take out a premier support agreement.

“If you have not migrated away from Microsoft XP, then you must urgently take out an agreement to continue to access critical and important security updates beyond 8 April for XP, Office 2003, and Exchange 2003.”

Microsoft launched Windows XP in 2001, and spent many months warning that it would end support for the operating system on 8 April 2014.

Despite this, the NHS – in common with the banking sector and other parts of the public sector, such as police forces – is a heavy user of the system.

Research by EHI Intelligence calculated that approximately 85% of the 800,000 PCs in the NHS were running XP in September last year, and the number is unlikely to have declined significantly because of the length of time trusts need to migrate to more modern operating systems.

Trusts that have made the move – usually to Windows 7 – have warned that the process can take two years, because of the number of PCs in use, and the number of clinical applications that only work with the OS.

In recognition of these issues, Crown Commercial Services agreed a deal with Microsoft to continue support for the entire public sector, including the NHS, for a year at the start of April.

However, it has stressed that the £5.5m deal should not be used to delay migration plans. “Agreements such as these do not remove the need to move off Windows XP as soon as possible,” a spokesperson said when the deal was signed.