Police trained in handling chemical, biological and nuclear material have launched a search of the Berkshire home of Russian oligarch Boris Berezovsky, hours after the fierce critic of the Kremlin was found dead.

The circumstances of the 67-year-old's death were unclear, though there were unconfirmed claims that the former power-broker of Russian politics had killed himself at the property in Ascot. Police said the death was being investigated as unexplained and the search for hazardous materials was a precautionary measure. Reports said his body was discovered in a bath.

Berezovsky's death comes only months after he lost a high-profile and personally disastrous court case against fellow Russian oligarch Roman Abramovich. He had accused the Chelsea football club owner of blackmail, breach of trust and breach of contract in relation to a Russian oil company.

After the claims were dismissed, he was ordered by the high court to pay £35m of Abramovich's legal costs. His financial difficulties were recently exacerbated after his former mistress, Elena Gorbunova, 43, claimed Berezovsky owed her £5m in compensation over the sale of their £25m residence in Surrey.

Berezovsky is said by friends to have become deeply depressed last year, not just because he lost to Abramovich, but because the damning judgment at the end of the case had so badly damaged his reputation.

The businessman thought highly of the British legal system – explaining in an interview with the Guardian six years ago that it had been one of the reasons he settled in the UK – and is said to have been distraught that the judge, Mrs Justice Gloster, described him as "dishonest", "unreliable" and even "deluded".

One close friend said Berezovsky had taken antidepressants last year and had checked into the Priory clinic for a brief period about four months ago while being treated for depression, adding: "He was very, very low. He talked about suicide. He would say to me: 'It's all over, it's all finished, there's no point in anything – the best thing that could happen to me is that I have a heart attack.'

"But I still don't believe he had the courage to take his own life – he loved life too much."

Following the loss of his fortune, Berezovsky was said to have been concerned about his ability to support his children. He had six children by three women: his two ex-wives and Gorbunova. While two of his children are now in their 40s, two are in their 20s and his youngest are aged 12 and 10. The businessmen had been mired in a series of legal disputes, and is said to have been worn down by his long battles with the Russian state, during which there were reports that he had faced a number of assassination plots.

Berezovsky, a Kremlin insider in the days of Boris Yeltsin, left Russia in 2000 after a quarrel with Vladimir Putin and has been the subject of an extradition order by Russia. He has appeared on Moscow's most wanted list since 2001 on charges of fraud, money-laundering and attempted interference in the Russian political process. A Russian court sentenced Berezovsky in absentia for embezzling $2bn from two major state companies. Moscow has repeatedly requested his extradition, though British authorities have not complied.

He had forged close contacts with a number of influential British figures around the time of the 1996 Russian election, which saw Yeltsin defeat a communist challenger, and was permitted to settle in the UK. In time, he changed his name to Platon Elenin, Platon being the name of a character in a Russian film based loosely upon his life. He was subsequently given a British passport in this name.

In 2007, Berezovsky said he had been told to leave by British police for his safety after the British security services claimed they had evidence of an assassination plot against him.

In the meantime, he led a lavish lifestyle, with properties in London, the home counties and the south of France, a private plane and a 1927 Rolls-Royce.

All that was wiped out by the disastrous defeat in the Abramovich case. Last week he was reported to have been selling a £50,000 Andy Warhol portrait of Lenin.

One Russian lawyer, Alexander Dobrovinsky, said that Berezovsky "was in a dreadful, horrendous state; covered in debt, almost broke".

According to Putin's official spokesman, Berezovsky wrote a personal letter to the Russian president two months ago, asking for forgiveness. "Boris Berezovsky passed a letter personally written by him. He admitted that he made lots of mistakes and asked Vladimir Putin to forgive him," said Dmitry Peskov. "He asked Putin for a chance to come back to Russia."

Peskov said nothing about Putin's reaction to Berezovsky's letter, adding: "I can say that in any case information about somebody's death – whoever the person was – cannot bring positive emotions." In the UK, however, friends of Berezovsky cast doubt on this claim.

In Russia, Berezovsky's death was greeted with surprise, with former associates and observers offering opinion on his legacy and the possible causes of his unexpected demise.

Opposition politician Boris Nemtsov, who was a rival of Berezovsky in the late 1990s and knew him well, said: "His life at the end was terrible … he lost money, he lost to Abramovich and he lost his wife … he not only suffered financially but also personally.

"His minuses for the country were more than his pluses, but he wasn't too much of a devil – although he often wanted to look like a devil."

Prominent opposition activist Sergei Parkhomenko, talking to liberal radio station Ekho Moskvy about Berezovsky's differences with Putin, said: "We were talking once and we turned to the strange topic of health and illness. He told me: 'My health is yet another weapon. Who dies first, loses. And I never lose'."

Parkhomenko added: "He didn't seem like a potential suicide."

Mark Feigin, a lawyer, who worked as a civil servant in the 1990s and met Berezovsky several times, said: "He was not a little person in politics and had unbelievable energy … things you couldn't do, Berezovsky could do.

"Over the last few months he had been experiencing a full exhaustion of his internal reserves, maybe this caught up with him."

Andrei Sidelnikov, a Russian emigrant, told gazeta.ru he had known Berezovsky for 11 years. He said: "Such a person could never commit suicide. He was an exceptionally joyous person and, despite his various reverses in life, always remained as such. My personal opinion is that his death has either natural causes, or [is the work of] the Russian secret service. Everybody knows that Boris was Putin's enemy number one."

RIA Novosti, a state-run news agency, cited a source close to Berezovsky, saying he had died of a heart attack, and had recently undergone treatment for health problems in Israel.

• This article was amended on 24 March 2013 to make clear that Berezovsky left Russia in 2000 rather than the 1990s, and because of a quarrel with Putin rather than claims of money laundering involving Aeroflot.