A computer virus, thought to be the first to attack Apple’s Mac OS X operating system, has been identified by internet security companies.

The Leap-A, or Oompa Loompa, virus is a potentially malicious program that is disguised as a simple image file. Experts are worried that its discovery will prompt hackers to mount the first serious campaign against Apple users.

The news is a blow to Apple’s reputation for security and to users of its Macintosh computers, which have long been regarded as far less vulnerable to virus attacks than PCs.

"Some owners of Mac computers have held the belief that Mac OS X is incapable of harbouring computer viruses, but Leap-A will leave them shellshocked," Graham Cluley, senior technology consultant for Sophos, the web security firm, said.

"Mac users shouldn't think it's okay to lie back and not worry about viruses."

He added that an online poll had found that most people thought the discovery of the Leap-A virus could lead to more attacks on Apple machines.

"The bad news is that most people think the situation is going to get worse for Macintosh users, and more threats will be targeted against the Apple community.

"The good news is that most don't believe it will ever be as big a problem as the one Microsoft Windows faces," Mr Cluley said.

The Leap-A worm spreads through Apple’s iChat instant messaging service, which is compatible with America Online’s popular AIM instant messaging program.

It is not considered a serious threat – partly because Mac users have to activate it by clicking on a file – but it can still disrupt the running of machines by stalling infected applications.

Sophos said that the program forwards itself to as a file called "latestpics.tgz" to contacts on the infected users' contacts list. The file then disguises its contents with a JPEG graphic icon in a move designed to fool people into thinking it is harmless.

"This is the first real virus for the Mac OS X platform," Mr Cluley said.

"Apple Mac users need to be just as careful running unknown or unsolicited code on their computers as their friends and colleagues running Windows."

Experts advise all computer users, whether running PCs or Macs, not to download files they are not sure about and to keep their anti-virus software updated.

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