A man has been questioned in connection with an alleged attempt to assassinate Russian tycoon Boris Berezovsky.
Mr Berezovsky says he was warned by sources in Russia and the UK
Scotland Yard said the man was arrested in central London on 21 June on suspicion of conspiracy to murder.
He was released without charge two days later into the custody of the immigration service.
Police released the information after the exiled Russian billionaire claimed UK intelligence officers had thwarted a plot to kill him.
Mr Berezovsky, 61, a critic of the Russian government, told the BBC earlier on Wednesday that he had been warned about the alleged plot by sources in Russia and Scotland Yard.
The Sun had reported that a Russian hitman had been hired to execute him at a London hotel. Russia's ambassador to the UK said he was not aware of any such plot.
The claims emerged as UK-Russian relations are strained in a row over a suspect in the Alexander Litvinenko murder case.
Mr Berezovsky, who lives in London, told BBC Radio Five Live he was told that "someone who you know will come to Britain, he will try to connect to you and when you meet him, he will just kill you and will not try to hide".
The killer would then say the murder was "just because of business reasons", Mr Berezovsky said.
"And in this case he will get 20 years, he will spend just 10 years in jail, he will be released, his family will be paid, he will be paid and so on," he added.
Mr Berezovsky later said he had been informed of the alleged plot by visiting friends from Russia and then by Scotland Yard three weeks ago. He claims the police advised him to leave the country for a week.
The Sun claims Britain's security services, MI5 and MI6, intercepted intelligence about the plot and detained a man.
The Home Office has refused to comment on what happened to the man after he was handed over to the immigration service.
BBC security correspondent Gordon Corera said it was unclear why he had not been charged.
Russia's ambassador to the UK, Yuri Fedotov, told BBC Radio 4's Today programme there was "nothing that could confirm" the plot.
Mr Fedotov later told the BBC that Britain's decision to halt contact with Russia's Federal Security Service would harm its fight against terror.
The Foreign Office would not confirm or deny links had been broken, saying it did not comment on matters of security.
Earlier this week, Britain expelled four Russian diplomats in the escalating row over the murder of ex-KGB agent Mr Litvinenko, who died in London last November.
Moscow has refused to hand over the man suspected of the murder - Andrei Lugovoi, another former KGB agent. Mr Lugovoi denies involvement.
KEY EVENTS IN CASE
1 November 2006: Alexander Litvinenko meets Andrei Lugovoi and another Russian at a London hotel
23 November 2006: Litvinenko dies in a London hospital
24 November 2006: A Litvinenko statement accuses Russian President Vladimir Putin of involvement in his death. Experts say Litvinenko was poisoned
6 December 2006: UK police say they are treating the death as murder
22 May 2007: Lugovoi should be charged with Litvinenko's murder, British prosecutors say
28 May 2007: UK makes formal request for Lugovoi's extradition from Russia
Russia says it is planning a "targeted and appropriate" response to the expulsions, adding that its constitution prevents it from extraditing its citizens to face trial in another country.
Mr Berezovsky, a Kremlin insider during the rule of Boris Yeltsin and who openly confesses that he is on a mission to bring down President Vladimir Putin by means of a bloodless revolution, blames Mr Putin for the murder of Mr Litvinenko, with whom he was friends.
Mr Berezovsky has survived numerous assassination attempts and has been a wanted man in Russia, charged with fraud and political corruption, since 2001.
Mr Litvinenko died of exposure to radioactive polonium-210 in London in November 2006.
The radioactive isotope used to poison him was found in several places that Mr Lugovoi had visited in London.