Sex offender Jeffrey Epstein reportedly planned to blackmail the Queen over allegations around her son Prince Andrew.
John Bryan – the US businessman who had a four-year relationship with Sarah Ferguson – has made the claim in an interview with the Mail on Sunday.
Bryan believes Epstein blackmailed powerful men into giving him money in exchange for his silence over their behaviour.
He says Epstein's "ultimate" aim was to blackmail the Queen but believes Prince Andrew never gave Epstein the "ammunition" to do so.
"It was just an extortion plot," Bryan told the Mail on Sunday.
"People always ask how Epstein made his money. He was supposed to be this tax wizard. But it was all a con.
"He blackmailed rich men and then made them pay to avoid scandal. He made hundreds of millions of dollars this way.
"Epstein tried to lure Andrew into his web but I believe his ultimate mark was the Queen."
Bryan says he believes Andrew's denials of any wrongdoing.
"I believe Andrew is innocent. If he genuinely was involved in 'orgies', as has been alleged, then Epstein would have used that to try and bribe the Queen into paying out millions to protect her family.
"Andrew has never had any money. The Queen was the one with money.
"I truly believe Epstein was going after her but Andrew never gave him the ammunition to do so."
Bryan says he was secretly called in for crisis talks with Andrew following his infamous interview with the BBC's Newsnight two years ago.
He claims he received a call from his former lover and Andrew's ex-wife, Sarah Ferguson, 10 days after the interview, asking him to come up with a strategic plan for dealing with its fallout.
"She invited me to come over. She was desperate. She told me that Andrew was in terrible shape," he said.
"He was distraught. They were distraught.
"In the four years I dated Sarah, I never once heard [Andrew] raise his voice or lose his temper," he said.
"It showed what strain he was under, how much he cared about the damage this was doing to the royal family."