Windows XP, the wildly popular operating system launched in 2001, will no longer benefit from security support or updates.

Microsoft announced the move to stop bug fixes on the platform last year, but fears remain that many small businesses have not received the message.

It is estimated that about one-in-five PCs still run XP, despite updates including Vista, 7 and, mostly recently, 8.

Dave Emm at security firm Kaspersky said: "Our data indicates that less than one fifth of our customers run Windows XP but more than a quarter of infections are Windows XP-based."

"Effectively, every vulnerability discovered after 8 April will become a zero-day vulnerability - that is, one for which there is not and never will be, a patch," he added.