"Apple has urged Mac owners to use anti-virus software. In a note posted on its support site in late November, Apple said it wanted to "encourage" people to use anti-virus to stay safe online.

The move is widely seen as a response to the growing trend among cyber criminals of booby-trapping webpages that can catch out Mac users.

Before now Mac users have been largely free of the security problems that plague Microsoft's Windows. The support note recommends that Mac owners install one or more of three anti-virus products.

Advice on the site said: "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 recommended users try McAfee VirusScan, Symantec Norton Anti-Virus 11, or Intego VirusBarrier X5.

The vast majority of malicious programs circulating online are aimed at Microsoft's Windows, largely because the software is used by so many people.

A handful of viruses have been written that targets Mac's OSX, but most have been demonstration versions only and few have had any significant impact on Apple users.

One virus, known as AppleScript.THT, could take control of a Mac and grab screenshots or keystrokes.

However, in recent months, hi-tech criminals have signalled a change in tactics away from e-mail borne viruses. Instead, many are infiltrating popular webpages in a bid to infect the machine of any and every visitor.

Many seek to steal valuable information such as login names, passwords or game accounts instead of trying to install themselves on a machine. "