A man was arrested on suspicion of conspiracy to murder in connection with an alleged attempt to assassinate the Russian tycoon Boris Berezovsky, Scotland Yard confirmed tonight.
The man, thought to be a Russian, was arrested in central London on June 21. He was handed over to immigration officials two days later, having been released without charge.
It is understood that he has since been deported. The Home Office refused to comment, saying it was a "security matter". Mr Berezovsky, 61, had claimed earlier that Scotland Yard warned him three weeks ago that a Russian assassin had been sent to murder him at the Hilton hotel on London's Park Lane.
The disruption of the alleged plot came before the UK's decision to expel four Russian diplomats, which was announced on Monday, and comes at a time of diplomatic crisis between the two countries.
The foreign secretary, David Miliband, said the expulsions were provoked by the Kremlin's refusal to extradite the former KGB agent Andrei Lugovoi, the prime suspect in the poisoning murder last November of the dissident former KGB spy Alexander Litvinenko.
Mr Berezovsky was a friend and sometime benefactor of Litvinenko.
Ministers are likely to have been informed by the police and intelligence services of the existence of a plot on the life of Mr Berezovsky, who fled to the UK in 2000 after falling out with the Russian president, Vladimir Putin.
Moscow has yet to announce details of its response to the UK's expulsion of diplomats.
Earlier today, Mr Berezovsky said in a statement: "I was advised by the police to leave the country if I could. I went overseas for a week and then the police informed me that I could return. I have been asked by the police not to go into detail about the assassination attempt and therefore I will not do so."
But at a press conference today Mr Berezovsky said he held the Russian president responsible for the alleged plot. "It's [Vladimir] Putin personally," he said.
The Sun newspaper, quoting MI5 and MI6 sources, reported that the assassin took a child with him to the hotel in an attempt to avoid raising suspicion. The paper said the security services had intercepted intelligence about the plot and seized the hitman before he could act.
The plan, the paper said, was to lure the businessman to a room in the Hilton and shoot him in the back of the head.
In an interview with the BBC, Mr Berezovsky dismissed a suggestion that he could be "paranoid" and said he felt safe because Scotland Yard was "very professional".
He said even before news reached him of the alleged Hilton hotel plot, a source in Russia had told him of a plot to kill him in London. The killer would say it was for "business reasons", would go to prison and would be out after 10 years, with his family taken care of, he claimed. The Russian ambassador to London, Yuri Fedotov, said he was not surprised that Mr Berezovsky should be targeted because he took "each and every opportunity" to expose himself as a public figure. However, he ruled out any Kremlin involvement.
The ambassador said he expected Moscow's formal response to the expulsions to come soon.
Mr Putin has been incensed by the UK's failure to extradite Mr Berezovsky to Moscow, where he has been charged with money laundering and attempting to foment a coup against the Kremlin.
A trial in absentia against Mr Berezovsky opened last week in Moscow. The billionaire, who fell out with the Kremlin and fled to Britain in 2000, is accused of embezzling millions of dollars from the state-controlled airline Aeroflot.
Separate Russian charges of conspiring to seize power were laid against Mr Berezovsky after an April interview he gave to the Guardian in which he called for a violent revolution in the country.
Today, Jim Murphy, the new Europe minister, told MPs on the foreign affairs committee that there was no chance of a Lugovoi-Berezovsky extradition swap.
However, asked if Mr Berezovsky could be prosecuted for promoting sedition against a foreign government on British soil, Mr Murphy warned that the Crown Prosecution Service would "continue to assess comments, or alleged comments...and if they are above the threshold for a prosecution, they would do so".
The businessman was given asylum in the UK four years ago amid claims of a Russian plot to kill him. On that occasion, according to Mr Berezovsky, the killer was to have struck with a fountain pen filled with poison at Bow Street magistrates court, where he and his lawyers were fighting his extradition to Russia.
The highest-ranking KGB officer ever to collaborate with British intelligence, Oleg Gordievsky, today said there were plots targeting Mr Berezovsky "all the time".
Mr Berezovsky yesterday repeated his accusation that Mr Putin had ordered Litvinenko's murder. He told BBC2's Newsnight: "I'm 100% sure that behind this murder is not just Andrei Lugovoi but Putin himself. That's the reason Russia protects Lugovoi, because they are protecting Putin."
· Additional reporting: Ian Cobain, Richard Norton-Taylor, and Matthew Tempest