Energy giants are forcing up to 85,000 households hit by a doorstep energy scam to pay twice for their electricity.
Victims who have bought seemingly legitimate power for a pre-payment meter will later get demands from their energy provider to stump up the cash again (see the Cheap Gas & Electricity guide).
These are often the most vulnerable households who cannot get a cheap online tariff due to credit problems.
Under the scam, criminals cold call and then dupe unsuspecting households into buying power as they have cloned hundreds of genuine keys used to top-up pre-payment meters, which work as usual.
In contrast, when debit or credit cards are cloned and used fraudulently, banks and building societies usually take the hit so consumers are not out of pocket.
The problem affects customers of all of the leading energy companies including British Gas, EDF Energy, Eon, nPower, Scottish Power and Scottish and Southern Energy.
These organised gangs, which often appear legitimate, sometimes offer seemingly-attractive discounts such as £50 of credit for the price of £25.
While consumers who use a cloned key will be able to use power as if it were genuine, Energy UK, which represents power firms, says companies can identify when a top-up is fraudulent.
Christine McGourty, Energy UK director, says: "Energy companies can detect fraud and you'll end up paying twice, first to the criminal and then again to your energy company, which can always detect when electricity has been used, but not paid for through the proper channels.
"It's essential you only top-up from recognised PayPoint or Payzone shops, or The Post Office."
Energy UK says those in hardship will be treated "sensitively" but insists most will be told to pay twice.
Archna Luthra, MoneySavingExpert.com consumer products analyst, says: "When your bank card is cloned, the bank usually takes the hit so you are not out of pocket.
"If energy firms cannot stop criminals cloning top-up keys, they should not force consumers to pay twice for their electricity.
"What makes the problem worse is those on pre-payment meters are often the most vulnerable members of society.
"The important message is to only buy your energy from an official source."
Energy firms say they never sell electricity top-ups door-to-door.
Companies have begun writing to and telephoning electricity pre-payment meter customers to alert them.
Police and the energy industry are working to track down the gangs and to help prevent this scam in future.
The authorities are also urging the public to hand over any evidence of fraudulent activity.
Dave Cording, Crimestoppers' operations director, says: "If anyone knows someone selling illegal electricity top ups, please contact Crimestoppers anonymously on 0800 555 111 or Crimestoppers-uk.org.
"By contacting Crimestoppers, no one will know your identity and you will not go to court. The public can play a vital role in helping to catch criminals, simply by taking a few minutes to pass on any information."
Mike OConnor, head of lobby group Consumer Focus, says: "This despicable scam is putting cash in the pockets of criminals and defrauding thousands of people who are already hard pressed.
"Consumers must be on the alert for anyone who comes to their doorstep claiming to sell electricity credit. Any customer worried they may have been affected should call their energy supplier or Consumer Direct to seek advice."
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