Murdered Russian ex-spy Alexander Litvinenko's "most likely poisoner" was former KGB officer Andrei Lugovoi, sources have told the BBC.
Mr Lugovoi has denied being involved in the poisoning
Mr Lugovoi met Mr Litvinenko on the day he fell ill, and radioactive polonium-210 has been found in a string of places he had visited in London.
But Mr Lugovoi has said he is a witness and a victim but not a suspect.
On Wednesday, Scotland Yard handed a file on the investigation to the Crown Prosecution Service.
But any suspect is unlikely to be transferred to the UK, as it is against Russia's constitution to extradite its own citizens.
Mr Litvinenko, 43, was a vehement critic of Vladimir Putin's government and there was intense speculation at the time of his death on 23 November that figures associated with the Kremlin might have ordered his death.
Moscow has denied any involvement.
The BBC's Frank Gardner said police sources privately believed that the "finger of suspicion" pointed "clearly" at Mr Lugovoi.
A trail of polonium-210 discoveries are believed to have roughly matched his movements:
- 16 October: Mr Lugovoi visited Parkes Hotel in Knightsbridge. Two rooms later found to be contaminated
- Mid-October: Meets Mr Litvinenko in Itsu sushi restaurant in Piccadilly
- 25 October: Mr Lugovoi stays at Sheraton Park Lane Hotel in Mayfair. Two rooms later found to be heavily contaminated
- 1 November: Meets Mr Litvinenko for tea at the Pine Bar at the Millennium Hotel in Mayfair along with two other Russians. Cup, teapot, bar and bar staff all believed to be contaminated
Investigators suspect that a phial of polonium-210 could have been tipped into Mr Litvinenko's tea.
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.
Alexander Litvinenko was a former Russian security officer
"What they don't know, and probably never will know, is who ultimately ordered the murder of Alexander Litvinenko."
When the claims were put to Mr Lugovoi he said he was innocent, adding: "As far as I'm concerned, I'm still a witness and not a suspect."
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 are now considering whether there is sufficient evidence to charge anyone over Mr Litvinenko's death.
At the end of last year, Scotland Yard officers travelled to Russia to question witnesses in the case.
As well as Mr Lugovoi, Mr Litvinenko, a former Federal Security Service (FSB) officer, met two other former Russian secret agents just before he fell ill in London - Dmitry Kovtun and Vyacheslav Sokolenko.
They have denied any involvement in his poisoning.