A scam in which criminals pose as police and ask victims to take part in a fake undercover operation is becoming a growing problem, according to a finance industry trade body.
UK Finance said it is receiving growing numbers of reports of the scam, with victims often losing large amounts of money.
The scam sees fraudsters contacting victims, usually by phone, purporting to be from the police, or in some cases the fraud team at their bank.
The criminal claims they are investigating a fraud at a local bank branch where staff are suspected of being complicit, including issuing fake bank notes, and asks the target to help in the operation.
As part of the scam, the victim is asked to visit the branch and withdraw a substantial sum, often thousands of pounds, of the supposedly counterfeit cash to hand over for "analysis".
The victim is assured the money will eventually be put back into their account, but the fraudster disappears with the cash.
The criminal tells the target not to discuss the case with anyone in the branch so, despite being questioned by bank staff, the victim takes out the cash, convinced employees are part of a fraud.
In another version of the scam, the criminal convinces the victim to transfer money to a "safe account" to protect the funds from "corrupt" bank staff, but the account is controlled by the criminal.
Katy Worobec, head of fraud and financial crime prevention, cyber and data sharing at UK Finance, said: "This is a particularly nasty scam as it plays on people's public-spirited nature to assist the police.
"We are receiving a growing number of reports of it occurring, with people often losing large amounts of money, so it's vital that everyone is aware.
"Remember, the police will never ask you to withdraw money and hand it over to them for safe-keeping."
Here are UK Finance's tips to avoid the scam:
1. 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
2. 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
3. If you are approached, or feel something is suspicious, hang up the phone and do not reply. Then report it to Action Fraud and your bank on their advertised number
4. Visit the Take Five to Stop Fraud website for more advice on how to stay safe from scams