A dark grey box appears on the screen

The screens on infected machines display a dark grey icon called Phage and go blank when the program is run. The virus then replicates itself to other PalmOS applications.

Security software companies were alerted to the problem at around 0100 GMT. They played down the risks and offered patches within hours for users to download.

Someone claiming to have found the virus on a website alerted the companies to its existence, so it is not certain the malicious program is really on the loose.

Legitimate target

"This isn't a big risk to Palm users because there aren't many, if any, copies of the virus out there in the wild," Craig Coward, of anti-virus company F-Secure, said. "But it is interesting because it is the first virus that specifically targets the Palm platform.

It is really a very unsophisticated virus - nothing that can be compared to the viruses we see on the PC

Graham Cluley, Sophos

"These handheld devices are becoming legitimate targets - it is a foretaste."

Any handheld device running the Palm operating system is potentially at risk. This includes machines made by Palm itself, as well as HandSpring, Symbol, TRG, and Sony.

Recently, security companies reported an "experimental" trojan program that attempted to delete files on Palm personal digital assistants (PDAs).

Infrared beaming

But the so-called Liberty trojan, which never got into the wild, had no way of replicating or spreading itself.

In that sense, Phage marks something of a step up. It is capable of spreading from one Palm application to another.

It can also spread from one machine to another if infected files are shared via infrared beaming or installed via a docking station.

But Graham Cluley, of the Sophos anti-virus company, said Palm users should not be unduly worried.

Safe computing

"As soon as you run an infected program, it blanks the screen and then infects another file," he said.

"But it is really a very unsophisticated virus - nothing that can be compared to the viruses we see on the PC. They haven't worked out very well yet how to write viruses for the Palm platform."

Mr Cluley said recovery from Phage was possible by resetting the PDA and restoring its contents from a backup.

"This virus is a very low threat to Palm users. Those PDA users who follow safe computing guidelines are highly unlikely to be seriously affected. It remains to be seen if this is the first of many such viruses on palmtop computers."