BT has launched a clampdown on rogue diallers, which redirect unwitting dial up modem users to expensive premium rate lines.

From May customers will be able to download BT Modem Protection, software that will alert users if their modem dials an unauthorised number. This will not bar malicious software from trying to redirect calls, but will let users stop it before it connects.

"If you use BT Modem Protection there's no reason why you can't immediately disconnect before it costs you a penny," said Gavin Patterson, group managing director at BT Consumer and Ventures.

"But we realise that prevention is only half of the solution. For those customers who may well be unaware of the costs that diallers can run up, the key thing is to give them the earliest possible warning that their bill has increased dramatically."

The company will also launch an SMS service that will warn customers if their bills rise sharply for any reason. This will include a telephone number that customers can call to block international or premium calls instantly.

There are no plans to charge for the service as yet, and both systems have been tested successfully in Northern Ireland.

Rogue diallers are one of the oldest scams on the internet. They only affect people using dial-up modems, which are hijacked by maliciously coded software and redirected to premium rate lines overseas.