Virgin boss Sir Richard Branson said a conman posing as Defence Secretary Sir Michael Fallon attempted to dupe him out of $5 million (€4.25m) after lying about a kidnapping.
The 67-year-old spoke to a man who "sounded exactly like Sir Michael" who said a British diplomat was being held hostage by terrorists and here was a "very sensitive reason" for wanting to get him back.
Writing about the incident on his website, Sir Richard said his assistant had received a note on what appeared to be official notepaper with a request to call the Defence Secretary.
He said: "He told me that British laws prevented the Government from paying out ransoms, which he normally completely concurred with. 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.
He wrote: "His secretary assured me that Sir Michael hadn't spoken to me and that nobody had been kidnapped. It was clearly a scam. I told them what had happened and we passed the matter over to the police."
The tycoon also wrote of a similar incident where a fraudster impersonated him and was able to con $2 million (€2.1m) of money meant to help victims of Hurricane Irma in the British Virgin Islands.
He wrote: "They told me that they had received an email from somebody claiming they were my assistant, to arrange a call with me.
"When the call happened the conman did an extremely accurate impression of me and spun a big lie about urgently needing a loan while I was trying to mobilise aid in the BVI.
"They claimed I couldn't get hold of my bank in the UK because I didn't have any communications going to Europe and I'd only just managed to make a satellite call to the businessman in America. The business person, incredibly graciously, gave $2 million, which promptly disappeared."
He added: "People used to raid banks and trains for smaller amounts - it's frightening to think how easy it is becoming to pull off these crimes for larger amounts."