A close friend of murdered ex-spy Alexander Litvinenko has broken his silence in a BBC interview.
Boris Berezovsky told Newsnight that Mr Litvinenko, 43, had said former KGB agent Andrei Lugovoi was responsible.
Mr Berezovsky said Mr Litvinenko, whose body contained radioactive polonium-210, had told him: "I think Lugovoi is involved in my poison."
UK police sources have told the BBC Mr Lugovoi was the "most likely poisoner", but he has denied any involvement.
On Sunday, Russian detectives investigating Mr Litvinenko's death asked the Home Office for permission to come to the UK to question Russian exiles in London including Mr Berezovsky.
The Metropolitan Police has said a decision on the request would be made in due course.
Mr Berezovsky who, like Mr Litvinenko, is a critic of Russian President Vladimir Putin, said he would be happy to talk to British and Russian authorities.
"Scotland Yard must question me about my knowledge of Alexander," he said.
"But Scotland Yard has never asked me to answer to the Russian prosecutor office.
"And moreover, me personally, I initiated this idea for Scotland Yard people and told them I am absolutely open to meet people from Russia if it helps to investigate Alexander's case."
Mr Litvinenko, 43, died in London in November after being poisoned with polonium-210.
His friends have accused the Kremlin of ordering his assassination in response to his criticism of President Putin, but the Russian government has rejected such claims.
Police sources have told the BBC that the "most likely poisoner" was Mr Lugovoi, who met Mr Litvinenko in London on the day he fell ill.
Mr Lugovoi said he had been a victim of radiation poisoning himself, accused the British media of "lies" and said he should be regarded as a witness and not a suspect.
Last week, Scotland Yard handed a file on its investigation to the Crown Prosecution Service.