British police investigating the death of exiled Russian oligarch and Kremlin critic Boris Berezovsky said on Sunday a search of his house by chemical, biological and nuclear experts had found "nothing of concern".

The 67-year-old who emigrated to Britain in 2000 after falling out with President Vladimir Putin was found dead in his mansion in the upmarket town of Ascot outside London on Saturday.

Police officers trained in detecting chemical, biological, radiological and nuclear (CBRN) material inspected the house as a precautionary measure, but have given it the all clear.

"I am pleased to say the CBRN officers found nothing of concern in the property and we are now progressing the investigation as normal," police superintendent Simon Bowden said.

He said Berezovsky's death remained "unexplained".

He survived one assassination attempt in 1995 in which a bomb decapitated his chauffeur, and openly expressed his fear that his life was in danger.

His friend and fellow Kremlin critic Alexander Litvinenko died an agonising death from radioactive poisoning in London in 2006, in what Litvinenko's widow has said was an assassination by Russian agents.

Berezovsky's wealth has diminished in recent years and last year he lost a bitter multi-million pound legal battle with fellow British-based oligarch Roman Abramovich, the owner of Chelsea Football Club.

Berezovsky had sought more than 3 billion pounds ($4.75 billion, 3.8 billion euros) in damages and accused Abramovich of blackmail, breach of trust and breach of contract in an oil deal.

Following his defeat in a London court, he was forced to agree to pay Abramovich 35 million pounds ($56 million) in legal costs, although there is speculation that the final bill will be far greater.

The judge in the case described Berezovsky as "an unimpressive, and inherently unreliable, witness".

Berezovsky was a close confidante of former Russian president Boris Yeltsin and one of a handful of businessmen who became billionaires following the privatisation of Russian state assets in the 1990s.

But he fell out with Yeltsin's successor, Putin, and fled Russia in 2000 just in time to escape arrest on fraud charges.

In London, Berezovsky became one of the Kremlin's most outspoken critics and is believed to have given financial support to a circle of exiled Russian critics that included Litvinenko.

Berezovsky's body was found by a bodyguard at his property and paramedics were called to the house at 3:18 pm (1518 GMT) but he was pronounced dead at the scene, the ambulance service said.

Unconfirmed reports say he was found in a bath.

Forbes' Russian-language website published an interview he gave to a journalist Ilya Zhegulev, in which Berezovsky said his "life no longer makes sense" and that all he wanted to do was return to Russia.

Zhegulev said the interview had taken place on Friday, but had not been recorded.

The tycoon's friend Demyan Kudryavtsev dismissed speculation that Berezovsky had killed himself.

"There are no external signs of a suicide," he told the Prime news agency in Russia.

"There are no signs that he injected himself or swallowed any pills. No one knows why his heart stopped."

Berezovsky's private life has also been turbulent in recent years. His divorce with second wife Galina Besharova in 2011 was dubbed one of the costliest in Britain, and there has been a more recent legal wrangle with his partner Elena Gorbunova.

Born on January 23, 1946, in Moscow, Berezovsky worked as an academic for nearly two decades before taking advantage of the perestroika reforms to make his fortune.

However, the fast-talking Muscovite with a taste for the high life fell foul of Putin's crackdown on the oligarchs' political independence. In 2003, Britain granted him political asylum.

After the news of Berezovsky's death emerged, Kremlin spokesman Dmitry Peskov said the oligarch had written to Putin a couple of months ago saying he wanted to go home.

"He asked Putin for forgiveness for his mistakes and asked him to obtain the opportunity to return to the motherland," Peskov told Russian state television.

Russian politicians and commentators shed few tears for Berezovsky on Sunday.

Communist Party leader Gennady Zyuganov said he had "no good words or high praise" for him.

He told the RIA Novosti news agency that Berezovsky "himself admitted at the end of his life that he had lived for nothing, ending up without family, motherland, money, or friends. And the finale was fully consistent with that."