A conman posing as Defence Secretary Michael Fallon attempted swindling nearly £4 million out of Sir Richard Branson, according to the entrepreneur.
The Virgin boss said that a man sounding "exactly" like Mr Fallon called him asking for money to help rescue a British diplomat being held hostage by terrorists.
Sir Richard was asked confidentially to contribute £3.8 million to the rescue fund.
But after calling Downing Street to verify the request, the businessman realised the call had been a fraud.
Sir Richard wrote about the incident on his website, where he revealed details of another scam involving himself being impersonated.
The 67-year-old said that six months ago his assistant received a note on what appeared to be official notepaper with a request to call the Defence Secretary.
Sir Richard then spoke with somebody who "sounded exactly like Sir Michael".
"He told me that British laws prevented the Government from paying out ransoms, which he normally completely concurred with," he said.
"But he said on this occasion there was a particular, very sensitive, reason why they had to get this diplomat back.
"So they were extremely confidentially asking a syndicate of British businesspersons to step in.
"I was asked to contribute $5 million dollars of the ransom money, which he assured me the British Government would find a way of paying back."
Sir Richard said he was "sympathetic" to the request, but wanted to carry out checks.
After he rang Downing Street and asked to be put through to Sir Michael's office he realised the truth.