Apple users are being warned about a virus which locks them out of their files until they pay a ransom.

Hackers have infected a number of Macs for the first time with malware dubbed KeRanger.

It was first discovered last Friday, when users began unwittingly downloading the programme when they tried to use software called Transmission, which is used to transfer data on BitTorrent.

The so-called ransomware lays dormant for three days after infecting a computer, then starts to lock videos, photographs and documents from the user.

Now Apple users are expected to get ransom demands from today unless they immediately install an updated version of the software.

Palo Alto Networks, which detected the threat, said attackers infected two installers of Transmission version 2.90 with KeRanger on the morning of March 4.

A statement said: "When we identified the issue, the infected... files were still available for downloading from the Transmission site.

"Transmission is an open source project. It's possible that Transmission's official website was compromised and the files were replaced by re-compiled malicious versions, but we can’t confirm how this infection occurred."

An Apple spokesman said the technology giant has also taken steps to prevent further infections, by revoking a digital certificate which had enabled the ransomware to be installed onto Macs in the first place.

Users of Microsoft Windows have for a long time been targeted by ransomware, with criminals generating millions of pounds a year from the scam.