A Russian exile, Nikolai Glushkov has been found dead at his home in London just over a week after a nerve attack on a former double agent.
The Metropolitan Police said an investigation is underway, but did not name Mr Glushkov.
It added that there was "no evidence to suggest a link to the incident in Salisbury".
Counter-terrorism police are investigating the death as a precaution because of the associations that the man, aged in his 60s, is believed to have had.
Former Russian spy Sergei Skripal and his daughter Yulia were poisoned by a nerve agent believed to have come from Russia.
He was found dead on Monday night.
Glushkov's death has done nothing to play down fears amid a highly charged political climate with commentators quickly linking it to Salisbury.
Officers are not naming him because formal identification is yet to take place. A post-mortem examination will be held in due course.More news: Supreme Court refuses to allow entry of foreign law firms
Glushkov said: "You have the deaths of Boris and Badri over a short period of time".
Glushkov, who worked for Berezovsky's companies Aeroflot and AvtoVAZ, was discovered by his daughter Natalya at his house in New Malden in southwest London on Monday, the reports said.
In March 2013, Berezovsky was found hanged in his ex-wife's home.
Police said he killed himself, but his friends were less certain and a coroner recorded an open verdict.
Glushkov had said at the time that he did not believe Berezovsky died of natural causes.
Glushkov received political asylum in Britain after serving five years in prison in Russian Federation for money laundering and fraud, the Guardian newspaper reported.