Apple is said to be "encouraging" Mac users towards anti-virus security software.

A message on Apple's support site reads: "Apple encourages the widespread use of multiple antivirus utilities so that virus programmers have more than one application to circumvent, thus making the whole virus writing process more difficult".

Apple recommends users try McAfee VirusScan, Symantec Norton Anti-Virus 11, or Intego VirusBarrier X5, two of which are available from Apple's online store.

Traditionally Macs have always been more secure than Windows-based PCs as malicious programmes tended to be aimed at the more numerous PC users, but cyber-crime is changing.

There are now more web-based viruses and trojans that can infect any unprotected computer that browses on to an "infected" web page, so Mac users are now more at risk too.