Nikolai Glushkov, 68, was discovered dead late on Monday at an address in Clarence Avenue in suburban south-west London.
News of Glushkov's death comes amid heightened tensions between the London and Moscow over the recent poisoning of a former Russian agent and his daughter.
The Metropolitan Police said there was no evidence that suggested a link to last week's poisoning of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury, England.
In a statement, British police confirmed they were investigating the death of a man in his 60s in southwest London, but did not identify him.
Glushkov told the inquiry that he bumped into Litvinenko in the office of Boris Berezovsky, a powerful oligarch and émigré who was found dead in 2013 on the bathroom floor of his home in Berkshire, on the western outskirts of London.
Russian Federation has dismissed accusations of any involvement in the Skripal case as "nonsense", saying it would only cooperate with the British investigation if it receives samples of the nerve agent believed to have been used.
Of the Skripal case, British Foreign Secretary Boris Johnson said, "the use of this nerve agent would represent the first use of nerve agents on the continent of Europe since the Second World War".
Another notable case involved Alexander Perepilichny, a Russian banker turned whistleblower who died during a jog near his home in Surrey.
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As a close associate of Berezovsky, Glushkov was linked to one of the most successful of Russia's oligarchs to emerge from the collapse of the Soviet Union.
Billionaire and business oligarch Berezovsky later ran afoul of the Kremlin when he publicly denounced Putin and brought a lawsuit against his ally, fellow billionaire Roman Abramovich.
"You have the deaths of Boris and Badri over a short period of time".
He added: "too many deaths [of Russian emigres] have been happening".
Attorney Andrei Borovkov told Russian media outlets that his client, Nikolai Glushkov, has died, but said he was unaware of the time and circumstances.
Glushkov once worked for various Berezovsky enterprises, including the flagship Russian airline Aeroflot and the LogoVAZ auto company.
He is accused of organizing a large-scale fraud scheme, embezzling money from Russian Airlines.
His friend Mr Berezovsky fled to London and they met up again when Glushkov was freed five years later and came to Britain.