Mobile phone users should beware of being bitten by Mosquito, a Trojan masquerading as a game but designed to send out premium-rate SMS messages without the user's consent.

One of the first examples of malicious software aimed at mobile phone users, the Trojan has been hidden in a pirated game that describes itself as Mosquito v2.0.

But while the mosquito-shooting game is being played, the Trojan makes the handset send out text messages at premium rates.

The Trojan can affect phones running the widely used Series 60 Symbian operating system.

Once active the Trojan copies itself onto \system\apps\Mosquitos\Mosquitos.app, and begins sending SMS messages.

Users of Nokia's forums have already complained at losing money because of the virus.

The model used by the hackers is very similar to the problem of rogue diallers installed on PCs, said Rachel Lashford, analyst at Canalys.

"Symbian's operating system is the most popular in Europe and hackers are going for the biggest target," Lashford told vnunet.com.

"It seems to me [hackers] can use similar sorts of coding [to that used on PCs] on Symbian phones - these high-end phones are open to that kind of coding."

Lashford said that hardware and software vendors and operators should advise users on how to prevent the spread of the Trojan, which is contained in a .Sis file.

Symbian said it had been aware of the issue for the last few days but had sought clarification before commenting.

"We needed to check which version of the game had been cracked before commenting. That being said, the cracked game does make it clear that it is a pirated version," said Peter Bancroft, spokesman at Symbian.