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Industry loss adjuster Charles Ingram, managing director of Clement Doherty Adjusters Ltd, told ATG that a sophisticated gang was targeting fine art and possibly jewellery dealers in a campaign that could date back as much as two years. Single losses have been as much as £200,000.

The scam involves cards that cannot use the chip and pin facility for clearance, with customers having to give a signature. The card is handed over and processed through the electronic payment machine, with an authorisation code sent to the machine by the card provider as confirmation that the card has not been reported lost or stolen.

The problem is that often the genuine card holder does not know their card has been cloned until the transaction shows up on their account later, by which time the fraudster has left with the goods, leaving the retailer out of pocket.

Mr Ingram, who is still dealing with new instances of this type of fraud, says that London is "continuously under attack" and that those involved are very convincing. "They seem to have knowledge of staff, objects and so on," he told ATG. The gang is so well organised that even though members have been caught in three cases, no goods have ever been recovered.

The simple solution is not to accept payment unless chip and pin can be used, says Mr Ingram.

"I don't know whether there has been a problem with other cards or in other places across the UK," he said, "but I have heard of other cases beyond those we have been dealing with."