A close Russian associate of late tycoon Boris Berezovsky has been found dead in London, the Guardian newspaper reported.

Nikolai Glushkov, 68, was discovered by his family and friends late on Monday, the newspaper said, citing his lawyer.

Newas of Glushkov's passing was made public by one of his friends on Facebook. 

Glushkov spent five years in a Russian prison on money laundering charges in the 1990s, having been jailed after Berezovsky fell out with Vladimir Putin and fled to London. 

He was released from prison in 2004 and subsequently also moved to the UK. Last year, a Russian court sentenced him to eight years in prison following a trial in absentia. 

Berezovsky was found dead in Berkshire in 2013, with police recording the cause of death as suicide. Glushkov believed his associate was killed, and had told the Guardian as much that same year. 

Relations between Russia and the UK are on a knife-edge, with British Prime Minister Theresa May accusing Russian operatives of having sought to assassinate a former Russian spy in exile through the use of a nerve agent. 

Russia says it had nothing to do with the attack on former double agent Sergei Skripal and has accused Westminster of provocation.