The customers said the money was taken from their cards, when they intended to pay in other ways. The system, which can by used for payments of up to £20, is only supposed to take money from the cards when they are around 2 inches away from the contactless terminal.
But two customers, who contacted the BBC’s Money Box programme , said the payments were taken when their cards were in purses which they were holding in their other hand well away from the device.
In both cases, the cards were charged as the shopper was attempting to make a standard “chip and pin” payment with a different card.
Marks and Spencer is the country’s leading user of contactless payment and has rolled the technology out to all 644 of its UK stores. It receives more than 250,000 payments a week using the technology.
Sources have revealed that the company has received five complaints of payments being made unintentionally since the technology began being rolled out last year.
However, it raises the possibility that there could be other cases, where customers may not noticed the money being taken.
Sandwich chain Pret a Manger also said it was investigating similar claims after a customer reported paying with a card that was more than 11 inches away from the reader.
A spokesman for M&S said it had investigated each case it had been notified about and refunded customers where double payments have been made.
She said: “We’ve had an overwhelmingly positive response from our customers following the introduction of contactless which helps make payments quicker and more efficient for transactions under £20.
“We’ve tested the system extensively and it is robust and fit for purpose. Almost 250,000 transactions a week are now taking place in M&S stores on Contactless.”
Contactless cards use technology known as near field communication, which uses a weak radio signal to read information off a chip held inside the credit or debit card.
Many banks have begun offering the technology and most new bank cards now contain the chips. All cards that carry the technology have a logo that features a series of curved lines.
To use a card, customers must hold it less than 2 inches away from the card reader.
But one of the customers who contacted Money Box, who would only name herself as Rosemary from Sussex, said her card had been around a foot away when the payment was taken.
She said: “I put my card into the reader and the assistant was asking whether or not I wanted cash back.
“Before I could answer, the transaction came up as complete and the till issued a receipt so I hadn’t put in a Pin at all at that stage.
“I queried it with an assistant and she looked rather puzzled and looked at the receipt and compared it to my card and realised that the numbers didn’t tally.
“The amount had been debited from a different card other than the one I put into the machine.”