The alleged plot involved killing Mr Putin on a foreign trip
Police in London arrested two men in connection with a suspected plot to kill Russian President Vladimir Putin, it is reported.
The Russian nationals were planning to have him assassinated while on a foreign trip, according to The Sunday Times.
Scotland Yard confirmed that anti-terrorist officers detained two Russian men on 12 October for five days "following allegations of offences under the Terrorism Act 2000".
A police spokeswoman said the unnamed men, aged 40 and 36, had been released without charge on Saturday.
She declined to comment on reports that the two detainees were freed on condition that they return to Russia.
The Sunday Times reported that police officers from SO13, the anti-terrorist branch, were alerted after receiving a statement from Alexander Litvinenko, a former intelligence officer in the FSB, the main successor organisation to the KGB.
He was granted political asylum in Britain several years ago.
Mr Litvinenko said that the two arrested men, one of them a former KGB agent referred to as Major P, were attempting to enlist support among Russian emigrants in London to help conspire to kill Mr Putin.
The newspaper reported that the two men were detained at the high-security Paddington Green police station in west London.
Mr Putin met Mr Blair during a state visit to Britain in June 2003
The newspaper said that one of the suspects outlined details of the alleged plot in a telephone call to Mr Litvinenko.
Fearing that he was being set up to be implicated in the plot, he immediately alerted the authorities.
He told the newspaper of a meeting with one of the alleged plotters: "Major P said [Putin] had to be overthrown because he was bankrupting the country and was going to put everyone in jail".
It was suggested at this meeting that Chechen separatists could use sniper rifles with which to ambush Mr Putin on one of his trips overseas.
Mr Litvinenko, speaking through an interpreter, told the Associated Press news agency that he was unsure if the plot was serious.