Clever fraudsters are posing as anti-scam officers to trick victims into handing over personal details for fake refunds.
Action Fraud said a series of letters are currently in circulation that claim to be from the National Fraud Intelligence Bureau.
The letters - which are being sent out to victims of fraud - are offering them the chance to have their money refunded.
They use the NFIB branding and the name of the City of London Police’s Commissioner, Ian Dyson, to appear credible.
But it asks for personal details to be sent to a South African bank and Action Fraud warned anyone who receives the letters to ignore them.
The organisation said: "The fraudsters are sending these letters so that they are able to gather bank details and defraud people who have already fallen victim to fraud."