Moscow: The husband of one of the Conservative Party's biggest donors 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.
Leaked files show her husband received $8m (£6.1m). The money 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".
Billionaire Mr 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".
Mr 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.
Mrs Chernukhin's donations to the Conservative Party began in 2012.
The majority - more than £1.5m - came after the $8m payment linked to Mr Kerimov was made to her husband on 29 April 2016, although it is not clear if any of that cash went to the Tories.