Andrei Lugovoi is said to have left a trail of polonium-210
Andrei Lugovoi, who is accused of killing ex-KGB agent Alexander Litvinenko says Britain wanted to portray him as a "Russian James Bond".
The Crown Prosecution Service says it has enough evidence to charge Mr Lugovoi with the murder of Mr Litvinenko.
But at a dramatic news conference in Moscow on 31 May, Mr Lugovoi protested his innocence and said he would fight to clear his name.
"They [Britain] think that they found a Russian James Bond that penetrates the nuclear facilities and, in cold blood, poisoning his friend, and at the same time poisoning himself, his friends, his wife and children," he said.
Mr Lugovoi claimed that either the UK's MI6 intelligence agency, the Russian mafia, or Kremlin opponent Boris Berezovsky had carried out the killing.
Mr Berezovsky denies any involvement in Mr Litvinenko's death and said Mr Lugovoi's comments suggested that the Kremlin was responsible for the murder.
"Everything about Mr Lugovoi's words and presentation made it obvious that he is acting on Kremlin instruction," he said.
Mr Lugovoi claimed that Mr Litvinenko was a British spy and that British special services played the main role in his death.
"Even if (British special services) hadn't done it itself, it was done under its control or connivance," said Mr Lugovoi.
The Foreign Office said that the matter was a criminal rather than an intelligence matter.
"A British citizen was killed in London and UK citizens and visitors were put at risk," a Foreign Office spokesman said.
Mr Lugovoi also claimed the British secret service tried to recruit him to provide "compromising information" on President Vladimir Putin.
Although the CPS has issued a formal request for Mr Lugovoi's extradition, Russian officials have said they will not hand him over because it would violate the country's constitution.
The former KGB officer, who now heads a private security firm, had tea with Mr Litvinenko at London's Millennium Mayfair Hotel on the day he fell ill.
Traces of the radioactive substance polonium-210, which caused Mr Litvinenko's death, have also been found in a string of places Mr Lugovoi visited in London.
In February sources told the BBC Mr Lugovoi was the "most likely poisoner".
A trail of polonium-210 traces have been found which are believed to have roughly matched his movements.
On 16 October Mr Lugovoi visited Parkes Hotel in Knightsbridge, where two rooms were later found to be contaminated.
In mid-October he met Mr Litvinenko in the Itsu sushi restaurant in Piccadilly, which was also found to have traces of the substance.
Days later he stayed at the Sheraton Park Lane Hotel in London's Mayfair where another two rooms were found with heavy contamination.
On 1 November came the meeting with Mr Litvinenko, along with two other Russians, in the Pine Bar at the Millennium Hotel, which investigators believe proved fatal.
A cup, teapot, the bar and bar staff are all believed to have tested positive for polonium-210.
Investigators suspect that a phial of polonium-210 could have been tipped into Mr Litvinenko's tea.
Mr Lugovoi has repeatedly denied any wrongdoing, suggesting that someone has been trying to frame him.
He underwent medical checks in hospital after traces of polonium-210 were reportedly found in his body.
He has also been questioned by Russian and British detectives in December as a witness.
Mr Lugovoi said he met Mr Litvinenko 10 years ago, but was not a friend or business partner.
He said his security work brought him into contact with Mr Berezovsky, who also knew Mr Litvinenko.