The 67-year-old, a former power-broker who helped Putin climb to the top of Russian politics before falling from grace, was found at his house in Ascot, 40km west of London.
Police said his death was “unexplained” and sent radioactive, biological and chemical experts to do tests in the house as they tried to piece together Berezovsky’s final hours.
Alexei Venediktov, editor of Russia’s Ekho Moskvy radio, said he had heard from unspecified sources that Berezovsky had died from heart failure.
Berezovsky had survived assassination attempts and said he feared for his life after he became one of Putin’s fiercest critics, repeatedly calling for him to be forced from office.
He was also a friend of Alexander Litvinenko, the former Russian spy who was poisoned with radioactive material in London in 2006, a murder that strained diplomatic ties between Britain and Russia.
However, associates said the man who personified the ruthless world of post- Soviet politics was depressed and may have committed suicide or had a heart attack after the stress of losing a $6bn (€4.6bn) court case to Chelsea Football Club owner Roman Abramovich.
British media said Berezovsky had given an interview to Forbes Russia magazine shortly before his death in which he spoke of wanting to return to Russia. He had lived in Britain since fleeing from Russia in 2000.
“I do not know what to do. I am 67 years old. And I do not know what to do next,” he said in the interview, according to extracts published in several British newspapers.
Police stood guard outside Berezovsky’s mansion. Inside, detectives were carrying out a thorough search of the house.
“The CBRN [chemical, biological, radiological, and nuclear] officers found nothing of concern in the property and we are now progressing the investigation as normal,” Superintendent Simon Bowden of Thames Valley Police said.
Berezovsky, perceived by Moscow as a criminal who should stand trial for fraud and tax evasion, was humiliated in 2012 when he lost a legal battle with former partner Abramovich over shares in Russia’s fourth- biggest oil company, Sibneft.
Putin’s spokesman, Dmitry Peskov, told state-run Rossya-24 television that Berezovsky had written to Putin and asked for help in returning to Russia.
“Some time ago, maybe a couple months ago, Berezovsky sent Vladimir Putin a letter he wrote personally, in which he acknowledged that he had made many mistakes, asked Putin’s forgiveness for these mistakes and appealed to Putin to help him return to his homeland,” Rossiya-24 quoted Peskov as saying.