Thames Valley Police has said there is currently no evidence to suggest anyone else was involved in the death of Russian businessman Boris Berezovsky.
The 67-year-old businessman was found dead at his Berkshire home yesterday by a bodyguard.
Specialist officers in chemical, biological, radiological and nuclear (CBRN) substances examined the property today before giving the all clear.
Detective Chief Inspector Kevin Brown, of Thames Valley Police, said: "It would be wrong to speculate on the cause of death until the post mortem has been carried out.
"We do not have any evidence at this stage to suggest third party involvement."
Scenes of crime officers are continuing a thorough investigation to determine the circumstances of his death after the CBRN experts had examined the country estate as a precaution.
Police said they had relaxed the cordon around the property in Mill Lane in Ascot.
Superintendent Simon Bowden said: "In light of the findings of the specially trained officers who carried out the CBRN examination as a precaution, the majority of the cordon which was put around the perimeter of the property has now been lifted.
"One small road block remains in Mill Lane outside the entrance to the property's grounds. The roads in the area are open and traffic is flowing freely.
"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 said the CBRN team was initially called in after the paramedic who declared Mr Berezovsky dead at the scene had their radiation detector triggered.
Mr Berezovsky, who had survived a number of assassination attempts, amassed a fortune through oil and automobiles during Russia's chaotic privatisation of state assets following the collapse of the Soviet Union in the early 1990s.
Once a member of Russian President Boris Yeltsin's inner circle, Mr Berezovsky fell out with Mr Yeltsin's successor, Vladimir Putin, and fled to Britain in the early 2000s to escape fraud charges that he said were politically motivated.
He became a strident and frequent critic of Mr Putin, accusing the leader of ushering in a dictatorship, and accused the security services of organising the 1999 apartment house bombings in Moscow and two other Russian cities that became a pretext for Russian troops to sweep into Chechnya for the second war there in half a decade.
Russia repeatedly sought to extradite Mr Berezovksy on a wide variety of criminal charges, and the tycoon vehemently rejected allegations over the years that he was linked to several deaths, including that of journalist Anna Politkovskaya and ex-KGB agent Alexander Litvinenko.
Last year, Mr Berezovsky lost a multibillion-pound High Court case against fellow Russian Roman Abramovich and was ordered to pay £35m in legal costs.