A new Trojan horse hitting Symbian 60 Series handsets makes the phone useless until it''s reformatted, deleting all the stored data, said a Finnish security firm Wednesday.

F-Secure said that the Fontal.a Trojan installs a corrupted font file into targeted handsets, causing them to fail at startup. Rebooting the phone by turning it off, then on, only compounds the problem, since the phone then essentially locks up.

"Currently the only known way to disinfect a phone that is infected with Fontal and is rebooted, is to reformat the phone, thus loosing all data on the phone," said F-Secure in its online advisory.

Because Fontal.a is a Trojan -- unlike a worm, it doesn''t propagate on its own -- the most likely way for users to infect their phones is to download files via IRC or P2P file-sharing to a PC, then install that malicious file on the phone.

Mobile threats are increasing not only in number, but in damage done. While earlier issues with cell phones have largely revolved around Cabir, and a new variant dubbed Mabir Monday, Fontal is one of the first to actually shut down the phone and force users to erase all data.

"An infection can be expensive," said Mikko Hypponen, F-Secure''s director of anti-virus research. "I have 1,700 addresses in my phone''s address book. Think how hard that would be to reproduce."

Symbian, the most popular operating system used by handset makers, is the most common target for mobile hackers. Symbian 60 Series is used in handsets made and sold by the likes of Nokia, Samsung, Siemens, Panasonic, and Lenovo.

F-Secure''s listing for Fontal.a can be found here.