Criminals may be using portable 'point of sale terminals' to steal money from contactless card holders as they walk past.
Contactless cards are now standard issue for new credit and debit cards, and account for one fifth of all UK card payments, according to the UK Cards Association.
They allow payments of under £30 to be made simply by holding the card near to the 'point of sale terminal'.
However, criminals are now reportedly buying the terminals and using the Near Field Communication (NFC) function on their mobile phones to read the data on nearby cards and secretly take payments.
The technique works when the machine is held within a few inches of the card, even when it's in a bag or a pocket.
Earlier this year, a post appeared on Facebook with a picture of a man apparently carrying out this scam in Russia - although there haven't yet been any verified reports of it happening in the UK.
The Daily Mail has carried out tests, and found that the card terminals were able to pick up payments from as far as four inches away. The scam worked best when cards were carried in a trouser or jacket pocket.
"We tested the machine with the card in a wallet, purse, bag and a pocket, and it read perfectly every time. The only exception was when the purse was inside a bag," the researchers say.
Fraudsters would, though, face some difficulty in getting started, as all card payments have to be made to retail accounts.
"There are thorough security checks before these can be set up and new accounts are monitored for any suspicious activity," says the UK Cards Association.
"Then, every card payment is fully traceable, right through to the recipient account. That means mean if any fraud were reported the recipient could be easily identifiable – and the money taken back."
However, it certainly wouldn't be impossible for a criminal to set up a legitimate business and then carry out the fraud on the side.
So how can you protect yourself?
One of the easiest safeguards is simply to have more than one contactless card in your wallet: if a point of sale terminal detects two, it flashes up a warning and doesn't take payment.
Alternatively, wrapping the card in metal - either a piece of tinfoil or a specially designed card holder - will block the signal.
And always check your bank statement carefully, and alert the bank if you spot any suspect payments - you will be refunded.