Businessman and politician Roman Abramovich gave Vladimir Putin a £25 million yacht in one of dozens of acquisitions through which the Russian president has amassed a vast covert fortune, it has been claimed.
A senior American official has openly accused Putin of corruption, nepotism, and direct embezzlement of state funds.
The US government has already imposed sanctions on Putin's aides, but it is thought to be the first time it has directly accused the president of corruption.
Putin's personal wealth has been the subject of speculation and claims of corruption for years, but little evidence has ever been produced to back up such allegations. Putin has dismissed claims about his wealth as "simply rubbish".
But a Panorama documentary the BBC broadcast this week detailed allegations that Putin has established a complex system of "training and practices" that allow him to "mask his actual wealth".
Details included Putin's acquisition of the 57m yacht Olympia. Dmitry Skarga, a former head of Russia's state shipping company Sovcomflot, said the boat was a gift from the oligarch owner of Chelsea Football Club, who transferred it to the president via an intermediary.
"It's a fact that Abramovich, through his employee, transferred a yacht to Putin," Skarga said. He also alleged that the yacht was maintained with state funds, even though it is Putin's personal property.
Adam Szubin, an official who oversees sanctions at the US Treasury, told the programme he believes Putin is corrupt and the US has been aware of this for "many, many years".
"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 said.
Abramovich was reputedly the first to recommend to former President Boris Yeltsin that Putin be his successor, in 1999. Abramovich would remain one of Putin's closest confidants; the president's biographer, Chris Hutchins, described the pair as having a relationship of father and favourite son.
Although Szubin did not put a figure on the size of Putin's fortune, he said the President's declared annual salary was "not an accurate statement of the man's wealth". In 2007 Stanislav Belkovsky, a Russian political commentator, claimed Putin had amassed $40 billion through stakes in three Russian energy companies. Last year, Bill Browder, a hedge fund manager and critic of Putin, claimed that the president was worth $200 billion - a sum that would make him by far the richest man in the world.
In 2014 the US stated that Putin had secret investments in the energy sector, but did not accuse him of corruption. Putin's spokesman has called these claims "complete trash".
The Kremlin told the BBC that the allegations did not require a response "as they are pure fiction".
Abramovich's lawyers called the claims about the yacht "unsubstantiated" and little more than a "rehash of speculation and rumours".