The details of debit and credit cards could be read by the scanner

The scanner, planted at an HSBC bank in Newbury, Berkshire, was able to record details held in the magnetic strips of cards, as people swiped them in the door to gain out-of-hours access to cash machines.

This allowed the thieves to transfer the details to other cards, a practice known as cloning.

Detectives are now hunting three men caught on CCTV footage installing their own camera at the bank in Mansion House Street, which police believe was used to watch people entering their personal details on the cash machine.

It has been a problem in the London area for some time, but several police operations there seem to have pushed the criminals into the home counties Pc Chris Baumgart

They also put "Out of Order" signs on other cash machines to encourage more people to use the machine in front of the camera.

The scanner was uncovered during a criminal investigation into credit and debit card fraud, Thames Valley Police said.

Pc Chris Baumgart, of the cheque and card fraud unit for west Berkshire police area, said: "We believe this is the first time this technique has been used in the Thames Valley.

"It has been a problem in the London area for some time, but several police operations there seem to have pushed the criminals into the home counties.

"Consequently we are asking the public to be wary of any suspicious-looking devices on the exterior doors of banks with lobbies which offer out-of-hours services and also of any paper signs.

"If a machine is out of order, the message will be displayed electronically on the machine's screen."

The three men are described as being of eastern European appearance and were all dressed in casual clothing.