Conmen are posing as police officers to dupe people into taking part in a fake undercover operation.
Members of the public are being phoned by someone posing as an officer saying they are investigating staff at a local bank branch.
As part of the scam, the individual is told to visit the branch and withdraw a substantial sum, often thousands of pounds, of supposedly counterfeit cash to hand over to the fraudsters for 'analysis'.
The victim is assured that the money will be deposited back into their account after the operation is complete.
However, once the money is passed over the fraudster disappears.
The scam has been designed to bypass bank security as the victims are duped into believing staff are involved in an alleged criminal activity.
As a result, despite being questioned about their withdrawal, the victim takes out the cash convinced that staff are part of the fraud.
In another version of the scam, the criminal convinces the victim to transfer money to a so-called 'safe account' to protect their funds from the 'corrupt' bank staff.
However, the account is in fact controlled by the criminal.
Detective Inspector Debbie King, from Derbyshire Constabulary, said: "This is a particularly nasty scam as the good-will and public spirit of people to help the police is preyed upon.
"We have seen similar scams in Derbyshire and I would urge anyone who believes they may have been targeted to please get in touch.
"All customers are reminded that the police and banks will never ask members of the public to become part of an anti-fraud operation or to transfer money to a ‘safe account’ for fraud reasons."
One Derbyshire woman was targeted by fraudsters stating that people had been posing as family members and attempting to purchase phones.
However, she became wary and after contacting the Metropolitan Police Service, where the fraudsters were purporting to be from, managed to avoid becoming a victim.
The 49-year-old said: "I got another phone call later in the day and I was thinking, 'I'm ready for you'.
"They asked me if I knew what was happening with my bank account yet.
"I said, 'yes, I do - and police are on their way to take a statement, so go and chew on a brick!'
"Then I put the phone down.
"I can see how people would fall for it.
"In the first conversation the man sounded very superior and knowledgeable.
"It was only the follow up that made me stop and think."
DI King added: "Often these scams are directed at individuals who are vulnerable and I would ask anyone with elderly relatives to make them aware of how fraudsters are operating.
"I would also urge people to register themselves and relatives on the mailing and telephone preference services.
"This is easy and free to do and provides another layer of protection against this type of fraud.
"However, it is critical that even those who are signed up to the services be wary of contact from fraudsters."
If you are concerned that you may have been a victim or that you have been approached by fraudsters, call Action Fraud immediately on 0300 123 2040.
TOP TIPS ON HOW TO KEEP SAFE
► The police will never ask you to become part of an undercover investigation or for you to withdraw cash and hand it to them for safe-keeping;
► Be wary of any calls, texts or emails purporting to be from the police asking for your personal or financial details or for you to transfer money;
► Make elderly and vulnerable relatives and friends aware of the types of calls to watch out for and register them for telephone and mail preference services;
► Hang up the phone, answer none of their questions and call 101 or 999 if someone is on their way to your home.