Anti-virus companies are warning Instant Messenger users to be wary of joke messages and e-cards as they may contain the Santa Claus worm.

IMLogic, a company that specialises in virus detection in Instant Messenger programs warned last night that the IM.GiftCom.All worm presented itself as a harmless Santa Claus file, apparently sent from someone known to the recipient.

Victims who clicked on a file link would see an image of Santa Claus but in the background the worm would be loaded on to their computer, IMLogic said. The worm would then send the same message to people on that person's address list possibly spreading like wildfire across the Internet.

The Santa worm is the latest tactic to be used on IM networks. Past tricks have included offers of movie clips to the latest release of "Star Wars" that instead led to an infected computer.

Anti-virus company Sophos said that this was just the latest such malware to be making the rounds as instant messaging and chat programs, provided by companies including AOL, Microsoft and Yahoo!, came under fire.

The anti-virus companies are worried because in the run up to Christmas a lot of people send joke programs - e-cards, Christmas cards and viruses play on that.