Researchers have found some of the most effective mobile malware to date: a family of Trojans that steal money directly from smartphone users.

The Viver family of Trojan horses targets Symbian phones and, once installed, sends premium SMS messages to a Russian mobile provider. Each message costs the user around £3.

The first mobile Trojans of this type were RedBrowser and Wesber, but they were Java-based, required user interaction and didn't have international dialling codes included, so only functioned in Russia. Viver has addressed these issues and has been distributed to thousands of mobile users via file-sharing sites, according to security experts.

Aleks Gostev, senior virus analyst at Kasperksky Lab, warned that the problem was likely to spread: "If there's money to be made, virus writers won't be slow to take up the opportunity."