Skype is being used by hackers to spread a "malicious worm" that infects Windows PCs and could result in computers being held to ransom.

The video chat service, owned by Microsoft, said yesterday it was "aware of this malicious activity" and warned users not to click on any "strange" or "unexpected" links.

A number of users received instant messages saying "lol is this your new profile pic?" By clicking on the link, they unwittingly downloaded a file containing Trojan horse malware.