Over the weekend, they received ten reports of fraudsters claiming to be Metropolitan Police officers in order to gain access to banking information and cards.
The fraudsters contact their victims by phone, claiming they have a suspect in custody who has been using the victim's bank card. They ask them to phone the bank and cancel the card, saying they will send a courier round to collect it.
Unbeknown to the victims, though, the fraudsters stay on the line and pretend to be staff at the bank. They hope to collect the card details during the call - and then collect the card itself.
"We received three calls around nine o'clock on Saturday, but fortunately none of these people had their cards or any money taken," says a police spokesman. "We just want to make sure people are vigilant, as we've had a spate of these incidents lately."
The police stress that no police officer or bank would ever request a customer to confirm their PIN number over the phone.
Such scams have been on the increase, according to Financial Fraud Action UK, which says one in 25 adults in the UK has been targeted. More than four in ten are over 50 years old.
"Always be wary of cold callers who suggest you hang up the phone and call them back. Fraudsters will keep your phone line open by not putting down the receiver at their end," advises DCI Dave Carter, head of the police's Dedicated Cheque and Plastic Crime Unit.
"Remember that it takes two people to terminate a call, so try and use a different phone line if you are asked to ring back. If you think you've already been a victim of this scam, contact your
bank or card company immediately."