A Russian businessman who was associated with a prominent critic of the Kremlin has mysteriously died in London, his lawyer said.

London's Metropolitan Police on Tuesday were treating the death as unexplained and have put counter-terrorism detectives in charge of the case "as a precaution because of associations that the man is believed to have had".

There was no evidence to suggest a link to the March 4 poisoning of former spy Sergei Skripal and his daughter, police said.

Attorney Andrei Borovkov told Russian media outlets his client, Nikolai Glushkov, died but said he was unaware of the time and circumstances.

Glushkov was an associate of Boris Berezovsky, a Russian oligarch and Kremlin critic who died in London in 2013. An inquest failed to determine whether he had killed himself or died from foul play.

Reports in British and Russian media said Glushkov, who was in his late 60s, was found dead at his home in southwest London.

Police were investigating the "unexplained" death of a man found at a house in the New Malden area late Monday, but did not release a name saying formal identification had yet to take place.

Glushkov had worked for various Berezovsky enterprises, including the car factory AvtoVAZ and flagship Russian airline Aeroflot.

He was arrested in 1999 and put on trial for embezzling $7m from Aeroflot. In 2004, he was sentenced to three years and three months in prison, but released because of time served before and during his trial.

Russian media reported Glushkov was granted political asylum in Britain in 2010.

In 2017, a Moscow court reviewed Glushkov's case and sentenced him in absentia to eight years for embezzling more than $122m from Aeroflot.

Last year, Glushkov appeared on a list published by the Russian embassy in London of Russians wanted for serious crimes that the UK had refused to extradite.

It said Russia had sought his extradition in 2015 "for committing a number of severe financial offences on the territory of Russia", but the British government refused.