Users can unwittingly run up huge phone bills

The scam often starts when a mobile phone user gets a message offering a prize if they call up to claim it.

The phone number turns out to be a premium rate line charging up to �1.50 a minute and the prize is often bogus.

To combat the scam, the TSI is calling on phone operators to allow customers to bar premium rate numbers for free.

Complaints double

According to the TSI, the scams are proving very lucrative to their operators.

The TSI has calculated that phone line fraudsters - who also trick people into calling using letters, small ads and bogus competitions - could be raking in as much as �1bn a year.

"We estimate that more money is probably being taken from consumers every day in premium rate scams than through any other scam," said Ron Gainsford, TSI chief executive.

It's like a sickness that is reaching epidemic proportions - and we are struggling to contain it Ron Gainsford, Trading Standards Institute Watchdog takes on rogue diallers

Complaints from people unwittingly dialling expensive and fraudulent premium rate numbers have doubled in the past year, the TSI added.

"It's like a sickness that is reaching epidemic proportions - and we are struggling to contain it," Mr Gainsford said.

The TSI gives the example of a fraudster who placed a small ad offering a cheap computer games console.

Callers who rang the number given were asked to hold on while a man fetched his son - after which several minutes passed with no further response.

Rogue diallers

They later found they had been dialling a premium rate number costing �1.50 a minute.

Rogue internet diallers - programs which divert dial-up internet connections to premium rate numbers without users' knowledge - are also a growing problem.

The TSI is calling on phone companies to allow their customers to bar premium rate lines for free.

Most firms already offer a number-barring service for free but a few operators charge.

The TSI also wants to make it easier for customers to identify which premium rate numbers - currently they are all prefixed 090 - are the most expensive ones.

The warning has been issued as part of National Consumer Week, which started on Monday.