People in St Albans and Harpenden are being warned about a sophisticated scam where victims are lured into giving bank details to conmen posing as police officers.
Hertfordshire Constabulary said residents were being urged to give up bank details by callers purporting to be police officers investigating a fraud on their account.
The victims are asked for account information, including their card numbers, security numbers and pin numbers.
If the resident becomes suspicious they are told to call 999 or 101 and ask for confirmation that the person is a police officer.
When the victim then calls police, the conman does not hang up so the call goes straight through to them again.
Officers said in some cases a female comes on the line pretending to work in the police control room and verifies the false officer’s details.
The caller is then transferred back through to the first caller who obtains all their details.
In some cases couriers have been immediately sent to the victim’s property to pick up their cards, which are then used fraudulently.
Hertfordshire Constabulary said the elderly are being particularly targeted with the scam.
Police have been receiving reports of this scam across the county since the beginning of July.
PC Peter Eames, from the Financial Institutions and Payment Card Crime Intelligence unit, said: "I am pleased to say that in the majority of these cases the resident refused to give their details. But these call show that the offenders are continuing to target people in Hertfordshire.
"In previous cases some residents have been conned into handing over their cards and pin numbers, resulting in thousands of pounds being removed from their accounts.
"I would reiterate the advice - if you receive a similar call do not to part with any details. Police would never ask you for this information. "I would urge people to share this information with their elderly relatives or neighbours so they are aware."