Scotland Yard has handed a file on the investigation into the death of the former Russian spy Alexander Litvinenko to the Crown Prosecution Service.
Alexander Litvinenko was a former Russian security officer
The 43-year-old, a critic of Moscow, died in London in November. His body contained radioactive polonium-210.
The BBC's Frank Gardner said police sources had told him that the "finger of suspicion" pointed "clearly" at former KGB officer Andrei Lugovoi.
But Mr Lugovoi said he had been a victim of radiation poisoning.
Mr Lugovoi has been at the centre of the investigation into the poisoning due to the radiation trail that has followed him.
Police have said they cannot reveal the contents of the file.
But Mr Gardner, the BBC's security correspondent, said Mr Litvinenko is thought to have consumed tea contaminated with polonium-210 while with Mr Lugovoi and two other Russians at the Millennium hotel on 1 November.
Our correspondent said the case against Mr Lugovoi was yet to be disclosed, but is thought to rest on forensic evidence.
He said: "The investigators believe they have now identified the probable poisoner.
"What they don't know, and probably never will know, is who ultimately ordered the murder of Alexander Litvinenko."
However, our correspondent said the chances of him being extradited are thought to be slim.
When the claims were put to Mr Lugovoi he said he was innocent and is also a victim.
"As far as I'm concerned, I'm still a witness and not a suspect, he said."
Mr Lugovoi went on: "When I do see or hear any official accusations, then I'll be happy to answer those.
"I consider myself to be a victim in this case because me, my friends and my family were attacked in the UK.
"And now it is up to the British prosecutors to establish how it happened and who did it."
Prosecutors will now consider whether there is sufficient evidence to charge anyone over Mr Litvinenko's death.
Mr Litvinenko is reported to have fallen out with the Russian leader Vladimir Putin in the late 1990s.
At the end of last year, Scotland Yard officers travelled to Russia to question witnesses in the case.
Mr Litvinenko, a former Federal Security Service (FSB) officer, met three other former Russian secret agents just before he fell ill in London - Mr Lugovoi, Dmitry Kovtun and Vyacheslav Sokolenko.
They have denied any involvement in his poisoning.
Mr Litvinenko's friends have accused the Kremlin of ordering his assassination in response to his criticism of President Vladimir Putin. The Russian government has rejected the claims.