The husband of one of the biggest donors of Britain's Conservative Party was secretly funded by a Russian oligarch with close ties to President Putin.
Lubov Chernukhin has given £1.7m to the Tories, including paying to spend time with the last three prime ministers, reports BBC.
Leaked files show her husband received $8m which initially came from a politician facing US sanctions due to his closeness to the Kremlin.
Her lawyers say the donations are not tainted by Kremlin influence.
A leak of banks' "suspicious activity reports" - called the FinCEN Files and seen by BBC Panorama - shows Vladimir Chernukhin was sent the money in 2016 from a British Virgin Islands company linked to Suleyman Kerimov.
Officials at Deutsche Bank in New York reported it as being among $278.5m of transactions involving the offshore company.
There has been an increasing focus on donations to political parties from wealthy UK-based Russians in recent years, with July's parliamentary report by the
Intelligence and Security Committee referring to the possibility they could allow people to "assist Russian influence operations".
An unusual donor in UK politics
Billionaire Kerimov is a member of the upper chamber of the Russian parliament. He has been under investigation in France over allegations of tax fraud since 2016.
In 2018 he was sanctioned by the US authorities, who were targeting those they said "play a key role in advancing Russia's malign activities".
Chernukhin, 52, is a former deputy minister of finance under Vladimir Putin, who left Russia for London in 2004 after being sacked by the president.
The Chernukhins - one of the UK's most prominent Russian-born couples - are now both British citizens and live in London.
Lubov Chernukhin's donations to the Conservative Party began in 2012.
The majority - more than £1.5m - came after the $8m payment linked to Kerimov was made to her husband on 29 April 2016, although it is not clear if any of that cash went to the Tories.
Journalist and Russia expert Edward Lucas, who gave evidence to the parliamentary inquiry into Russian influence, told BBC, "The Chernukhins, pleasant people that they might be… are not fit and proper people to make donations to a British political party."
"I am profoundly concerned by the access that, not only Lubov Chernukhin but also other rich Russians have to the heights of the Conservative Party, and to the government," he added.
Chernukhin's donations to the Tories have given her access to figures at the top of UK government.
In return for £135,000 she was invited to a ladies' night dinner at a luxury hotel with Prime Minister Theresa May's cabinet in April 2019.
And the 47-year-old has twice made winning bids at auction for tennis matches with Boris Johnson. The last, in February this year, cost her £45,000.
In 2014 she paid £160,000 to play tennis with Prime Minister David Cameron and then London Mayor Johnson.
As of this year she is the biggest female donor in British history to the Tories. According to Electoral Commission records, she has given about £1.7m in total over the past eight years, including £335,000 between last January and July.
When in 2018, Boris Johnson - then foreign secretary - was asked about the Chernukhins and the £160,000 tennis match donation, he told the in a BBc show, "If there is evidence of gross corruption in the way that gentleman… obtained his wealth... then it's possible for our law enforcement agencies to deprive him of his wealth."
But he added "all possible checks have been made and... will continue to be made" on donations.
How the truth got out?
The $8m payment to Chernukhin was made by an offshore company called Definition Services, which was controlled by Kerimov's children. The documents show their funding came from their father and it was Mr Kerimov who had the personal relationship with Mr Chernukhin.
As Deutsche Bank was processing the money, it sent questions to officials at another bank involved in the transaction.
They said it was a "loan between the two parties aiming to support further immovable property projects of the borrower" and that Definition was involved in real estate investments.
Despite the response, Deutsche Bank filed a suspicious activity report (SAR), noting the link to Kerimov and the payment to Chernukhin.
Deutsche said Definition was "registered and banking out of high risk jurisdictions and the commercial purpose of the transactions and the relationship between the parties could not be determined".