And while a spokesman for the Association of Payment Clearing Services (Apacs) said the fraud had only occurred at Shell petrol stations, an INQUIRER investigation reveals that other outlets could also be susceptible to the crime.

According to our source, a team of shysters has been turning up at petrol stations posing as engineers and taking the Trintech Smart5000 Chip and Pin units away for repair. They have then bypassed the anti-tamper mechanisms and inserted their own card skimmer.

The hoods then return the unit, again posing as an engineer. Once the units begin collecting card details these are sent abroad and used to withdraw cash.

And our source warns the fraud could take place at any site, with any Chip and Pin terminal and trusting staff.

It is impossible for members of the public to distinguish a doctored unit from a standard chip and pin card reader, as the skimmer is inserted inside the unit, unlike with cashpoint card skimmers.

To get around the anti-tamper mechanisms, the fraudsters might have had access to a reset program that would allow them to reset the alarm or they were able to engineer their way round it by using different parts from previous versions of the Smart5000 unit. "Either way," said our mole, "they were very clever."

Eight people from the south of England have so far been arrested in connection with the scam and the plod are continuing their investigations. µ