A newly intercepted malicious Trojan has begun infecting mobile phones which run the Symbian operating system.

Dubbed 'Skulls' the Trojan has been detected on Symbian shareware download sites where it bills itself as 'Extended Theme Manager' by 'Tee-222'.

However, activation of the malicious code replaces all program icons with skulls and blocks them from running.

"If you see it, don't install it on your phone," said Mikko Hypponen, director of antivirus research at F-Secure.

"It will make the smartphone features of your phone useless. You can still make calls with the phone but that's it - no messages, no web, no applications.

"Recovery could get tricky, especially if you don't have third-party file manager software already installed on your phone."

While not strictly a virus, since it does not reproduce itself or spread, this kind of code is likely to become an increasing problem as downloading smartphone applications becomes more popular, Hypponen warned.