Telecoms giant BT took action against so-called rogue diallers following complaints from more than 80,000 of its customers.

The diallers are programs which hijack modems and dial up a premium rate number when users log on.

Users do not notice the scam until they receive their phone bill which can be thousands of pounds more than normal.

But free software and an early warning system provided by BT is starting to alleviate the problem.

Rogue diallers can find their way onto PCs in a variety of ways.

Some legitimate sites, often selling pornography, software or music, use premium numbers as a means of payment rather than requesting the user's credit card details.

Fighting back

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Some people misunderstand the terms and conditions involved when they download the free software needed to activate the content.

But others find themselves with an unwanted dialler as a result of them being hidden in other programs.

Sometimes they exploit security flaws in web browsers and operating systems to install the diallers secretly and, in other cases, the diallers are installed by programs hidden in spam e-mail or web pop-ups.

BT has been forced to confront the problem head on since receiving more than 80,000 customer complaints about the issue.

From the beginning of May, it offered its customers free software which warns them if their computer dials a premium rate or international number.

According to BT, 2,000 customers a day are downloading the software.

"We are pleased that up to 83 people an hour are protecting their computer from the threat of a dialler problem," said Gavin Patterson, BT group managing director.

BT is also providing a text message service which alerts people if their phone bill rises sharply.

Customers receive the warning either via their landline phone if it is text-enabled or, if not, via a voice message.