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LONDON - Police said Wednesday they had arrested a man suspected of plotting to kill Boris Berezovsky, the exiled Russian tycoon and vehement Kremlin critic who is one of the key figures in the escalating tensions between Moscow and London.

The Metropolitan Police said the man was arrested June 21 and turned over to immigration authorities two days later. The police did not further identify the man and British immigration officials declined to comment; the Russian Embassy said it had not been notified of such an arrest.

The police statement came hours after Berezovsky said he had fled the country for about a week in mid-June after police warned him his life was in danger.

Berezovsky was a close associate of Alexander Litvinenko, a former KGB agent who was killed in London last year with a dose of the radioactive isotope polonium-210. Litvinenko, also a harsh Kremlin critic who had received asylum in Britain, alleged in a deathbed statement that Russian President Vladimir Putin was behind his poisoning.

Britain named Andrei Lugovoi, a Russian businessman and former KGB agent, as a suspect in the Litvinenko murder and demanded his extradition. Russia refused, saying it is constitutionally prohibited.

Britain on Monday said it would expel four Russian diplomats in response to the extradition refusal, and Moscow threatened unspecified strong measures in return. The dispute marks a new low in Russia-Britain relations, which already had been troubled by Russia's opposition to the war in Iraq, by Britain's refusal to extradite Berezovsky to face embezzlement charges and by Moscow's allegation last year of spying by British diplomats.

The alleged plot against Berezovsky is likely to increase widespread suspicion that Russian agents are aiming to wipe out prominent political foes abroad. Russian agents killed a top Chechen separatist leader in 2004 in Qatar, and Russia later passed a law authorizing its forces to act against enemies overseas.

"I am happy that the British are very strong in protecting people," Berezovsky told the British Broadcasting Corp. "I don't have any chance to be alive, if not for the protection of the state which gave me asylum."

There was no confirmation that the alleged assassination plot was connected with Berezovsky's dissident views. He is one of the richest of the so-called "oligarchs" who amassed gargantuan wealth in shadowy privatization deals after the breakup of the Soviet Union.

Berezovsky says he would be willing to face the Russian charges against him if the trial were held in a neutral country's court, and he has suggested Lugovoi consider a similar arrangement. Russia has made no official response to that idea and Britain openly dismisses it.

"We want the trial to be in a British court, on British soil," said Michael Ellam, the spokesman for Prime Minister Gordon Brown.

Europe Minister Jim Murphy told the Foreign Affairs select committee Wednesday that Britain had made a targeted and measured response to Russia.

Britain's refusal to extradite Berezovsky, who was granted British citizenship after fleeing Russia, has long angered the Kremlin.

The Foreign Office said in a document Wednesday that relations with Moscow have been "overshadowed by tensions" over asylum granted to Russian dissidents.

Moscow has not "fully accepted that these questions are matters of law, not of politics or diplomacy," said the document, prepared by officials as part of a parliamentary inquiry into Russian-British relations.

Berezovsky, a one-time Kremlin insider who has fallen out with Putin, said Wednesday he fled Britain briefly last month because British intelligence services told him his life was in danger.

"I was informed by Scotland Yard that there was a plot to kill me, and they recommended to me to leave the country," Berezovsky told The Associated Press. He said he left Britain for about a week and returned when informed the plot had been foiled.

Berezovsky was granted political asylum in Britain in 2003. His visibility has increased since Litvinenko's murder.

Scotland Yard confirmed Berezovsky's remarks, saying they had arrested a man on suspicion of conspiring to murder the tycoon on June 21. Police said the suspect was handed over to immigration officials two days later.

"Berezovsky is a very high-profile critic of the Putin regime, and history does show that it would appear that the Russians are prepared to take action against their critics abroad," said a MI5 domestic intelligence agency official, who demanded anonymity to discuss sensitive intelligence work.

The official could not say whether British intelligence services believe Russia has tried to attack dissidents in London since Litvinenko's murder. But the official confirmed that about 30 Russian spies are believed to be based in London to monitor exiles in the city.

Russian Ambassador Yury Fedotov told BBC radio said the alleged plot to assassinate Berezovsky was "quite strange information, and I have nothing that could confirm it."

He alleged Berezovsky is linked "to many criminal international schemes of money laundering, corruption and organized crime."

Berezovsky said he first learned of the plot through contacts within Russia's Federal Security Service, the successor agency to the KGB. "They told me that someone I knew would come and kill me openly and present it as a business matter. He would say there was a disagreement over the business," he said.