Action Fraud is warning people to be wary following reports of a letter claiming to be from the National Fraud Intelligence Bureau (NFIB).
The letter is being sent to victims of fraud offering them the opportunity to have their money returned.
To receive the money, it asks them to send their personal details to a South African bank.
However, it uses the NFIB branding and the name of the City of London Police’s Commissioner to appear credible.
A spokesman for Action Fraud said the scammers are sending these letters so that they are able to gather bank details and defraud people who have already fallen victim to fraud.
City of London Police’s Detective Chief Inspector Andy Fyfe said: "This fraudulent letter is clearly not something that the National Fraud Intelligence Bureau would send to the public.
"It takes advantage of peoples’ trust in order to steal money from those who have already fallen victim.
"By using recognised organisations, including the National Fraud Intelligence Bureau’s branding and the City of London Police’s reputation, the fraudsters are attempting to appear credible.
"If you are unsure about a letter you have received from the National Fraud Intelligence Bureau, please contact Action Fraud directly before giving away any of your personal details."