The devices are hidden in a cover and attached to the top of cash points, where they record as customers enter their PIN numbers.
A false card slot is also attached, retaining the customer’s credit or debit card, which is then used by the thieves to withdraw cash using the PIN number.
Police in Harrow say the relatively simple trick is being used by an organised gang of criminals operating across the country to dupe unwitting customers out of huge sums of cash.
One of the devices, which are attached overnight, was recently found on an ATM at Barclays in Bridge Street, Pinner .
It was removed and police said the machine will soon be replaced by the bank with one of the tamper-proof ATMs already widely used elsewhere.
But they have warned customers to protect themselves by ensuring they shield the keypad at all times when entering their PIN number.
'Always cover your pin when using an ATM'
Sergeant Roni Doyle, the neighbourhood sergeant for Pinner South, said it was not the first time such a device had been found on the cash machine in Bridge Road.
She said police had identified one of the people responsible, who is believed to be part of a national criminal network, and were working to arrest him.
"These devices will not be used soon as most of the banks have already changed their ATMs to the new tamper-proof machines," she said.
"But customers should always cover their PIN when using these machines.
"We have identified this suspect and he will be arrested ASAP."