US Treasury official Adam Szubin claims that Russian president Vladimir Putin is corrupt and that the US government was aware of this for "many, many years". Szubin, who oversees US Treasury Sanctions, told BBC Panorama: "We've seen him enriching his friends, his close allies, and marginalising those who he doesn't view as friends using state assets.

"Whether that's Russia's energy wealth, whether it's other state contracts, he directs those to whom he believes will serve him and excludes those who don't. To me, that is a picture of corruption."

Szubin refused to be drawn on a secret CIA report in 2007 that credited Putin's fortune at around $40bn (£28bn). However, Szubin did say that Putin probably had much more personal money than his remuneration as Russian president.

"He supposedly draws a state salary of something like $110,000 a year. That is not an accurate statement of the man's wealth, and he has long time training and practices in terms of how to mask his actual wealth."

The Panorama documentary Putin's Secret Riches, which aired on BBC One on 25 January, alleges that Putin has created an intricate method of "training and practices in terms of how to mask his actual wealth."

Putin himself has categorically denied such allegations. "It's simply rubbish. They just picked all of it out of someone's nose and smeared it across their little papers."

Former Russian insiders say they have personal knowledge of Putin's financial situation. Dmitry Skarga, who used to run the state shipping company Sovcomflot, says he oversaw the transfer of a $35m (£25m) yacht to Putin. Skarga says the 57m-long Olympia was a gift from billionaire Chelsea football club owner Roman Abramovich, who transferred it to the president via an intermediary.

"It's a fact that Mr Abramovich, through his employee transferred a yacht to Mr Putin," Skarga told Panorama. The former shipping company boss also claims that the yacht is run and maintained with state funds, even though it belongs to Putin.

Bill Browder, a hedge fund manager and critic of Putin, claimed that the Russian president is worth $200bn (£140.3bn) - a sum that would make him one of the world's richest men.

"Putin is a very scared man," Browder said in an interview with Cbc News. "He's stolen a lot of money over the last 14 years — and then he saw a similar kleptocrat, Viktor Yanukovich, the president of Ukraine, be run out of the country by his own people for doing the same thing."