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

"I am pleased to say the CBRN [chemical, biological, radiological and nuclear] officers found nothing of concern in the property and we are now progressing the investigation as normal," said superintendent Simon Bowden of Surrey Police, the upmarket area outside London where Berezovsky's body was found at his mansion on Saturday.

Berezovsky, a Russian tycoon and former Kremlin insider who became one of President Vladimir Putin's fiercest critics, was found dead at his home near Ascot, a town near London, on Saturday afternoon.

"I would like to reassure residents that we are confident there is no risk to the wider community" - Stuart Greenfield,

Superintendent

His lawyer said the 67-year-old had been depressed over his debts and it was likely suicide, but police have cordoned off the mansion and surrounding streets as they investigate a death they are describing as "unexplained".

"A full inquiry is under way," the Thames Valley Police said in a written statement.

Berezovsky was friends with another Russian exile and Kremlin critic, Alexander Litvinenko, who died of radioactive poisoning in London in 2006. His widow blames the Russian state.

Superintendent Stuart Greenfield said police were taking "all necessary measures to ensure a full and thorough investigation can be carried out" into Berezovsky's death.

"I would like to reassure residents that we are confident there is no risk to the wider community," he said.

Berezovsky settled in Britain more than a decade ago after going into "self-imposed exile".

He has already been convicted and jailed in absentia by Russian courts on embezzlement charges.

On Saturday, following the news of his death, a spokesperson for Russian President Putin said that Berezovsky had asked the president for forgiveness and permission to return to Russia shortly before his death.