Internet users have been warned to watch their telephone bills after a man was charged £400 for an international call he did not make.

Paul Johnson, a 36-year-old sales rep from Cannock, Staffordshire, was shocked to receive a bill for £600, with £400 worth of calls to the south Pacific island nation of Tuvalu - even though he had never knowingly called there.

Staffordshire Police said Mr Johnson appeared to have been a victim of a growing problem, known as rogue dialling.

Mr Johnson said: "The first I heard about it was when I got a call from the high accounts department at BT and I was gobsmacked.

"I told them I had no idea what was going on so they said they would put a block on international calls and then I got a bill for £600 for the first quarter."

Staffordshire Police have warned people that they could be inadvertently dialling up phone bills worth hundreds of pounds as a result of rogue dialling.

A number of other people across the county had found their dial-up connections diverted to premium rate or international numbers without their knowledge or consent, a spokesman said.

Pc Steve McGrath, a creditcard crime investigator who has received several complaints about rogue dialling, explained how it worked.

He said: "While surfing on the internet, the unsuspecting victim receives a message similar to 'to view this page you must download the following software plug in'.

"They then click on this message so that they can view the information. The software changes the settings on the computer and the next time the computer is logged onto the internet it rings a premium rate or international phone number.

"The phone provider can insist that the bill is paid as the call was made from that particular phone. Failure to do so can result in people being cut off."

Mr Johnson contacted The Independent Committee For The Supervision Of Standards Of Telephone Information Services who confirmed he could have been the victim of a rogue dialler.

But he said he was angry with BT for not doing enough to protect his land line and had refused to pay the bill.

He said: "It's frustrating, I have had the corporate line that "you should have more security" but I think it should be the other way around.

"They haven't listened to me at all.

"If I pay it it's almost as if I have agreed with the bill. It's like somebody coming to your front door and getting you to sign for something you don't even want.

"They are just ignoring the problem. Some people will be ashamed to admit it because they think it's a little bit seedy when it's not, but it takes people like me to stand up to them.

"I'm not being martyrish but the more people that are aware of it gives us a chance to get our money back."

A spokesman for BT said: "We have been working with Ofcom and Icstis and we have actually managed to bar access to thousands of these lines in the last few months, but the problem is once you close down one another one opens somewhere else.

"It is an industry-wide problem and we have to work within the existing legal framework if we want to stop it," he said.

"We want to work with Mr Johnson, not against him, and we would advise him to pay the existing portion of his bill that is not in dispute and we can wait until an investigation sorts out what happened with the other calls."