The diplomatic row between Britain and Russia threatened to deepen further last night after Scotland Yard revealed that an assassin, supposedly sent to Britain to murder dissident tycoon Boris Berezovsky, was arrested and deported back to Moscow.

The unidentified man, who was seized in central London on June 21, was handed over to immigration officials two days later, the Metropolitan police said.

The dramatic twist in the post-Cold War saga of ex-KGB spies and murder on the streets of Britain comes amid the continuing controversy over the poisoning of Alexander Litvinenko, the former Russian agent turned British citizen, and the Kremlin's refusal to extradite the prime suspect of his murder Andrei Lugovoi, another ex-KGB spy, who denies any involvement.

No 10 and the Foreign Office were last night still awaiting Russia's response to the expulsion of four of its diplomats by the government in protest at Mr Putin's refusal to extradite Mr Lugovoi for trial in Britain.

Mr Litvinenko died last November after consuming radioactive Polonium 210. He fell ill after meeting Mr Lugovoi and another Russian businessman at a London hotel. A waiter claimed the poison was sprayed into a teapot. Mr Litvinenko, in a deathbed statement, blamed Mr Putin for his murder.

Scotland Yard's latest revelation came after Mr Berezovsky - a fierce critic of the Putin government - revealed how detectives had tipped him off about the assassination plan. This resulted in him leaving Britain on June 16 for a week until the Met contacted him to say the plot had been foiled and it was safe for him to return.

Last night, the 61-year-old Russian billionaire said: "I am happy that Britain is very strong in protecting people in this country. It is very strange that British government protects a Russian citizen in London and, to the contrary, Russia tries to kill a Russian citizen in London."

Earlier, Mr Berezovsky told a news conference how he had been informed three months ago by friends visiting from Moscow that the FSB, successor to the KGB, was "creating a plot to kill me".

The would-be hitman was supposedly due to travel to London, meet the emigré tycoon and kill him, citing "business reasons".

"He will spend 10 years in jail, will be released, will have a lot of money, and will be a hero of Russia," said Mr Berezovsky, adding: "I didn't take it seriously but what happened later is exactly what they said."

It is believed the alleged killer planned to murder his victim at the Hilton Hotel in London's upmarket Park Lane. The would-be assassin planned to lure Mr Berezovsky to a meeting and was to have taken a child with him so he looked less suspicious. The plan supposedly was for him to shoot the billionaire for being "an enemy of Moscow".

Mr Berezovsky said the Russian premier was moving towards a totalitarian system. "The mentality of almost everybody who works for this organisation is criminal," he claimed, stressing the people who killed Mr Litvinenko were those who plotted his murder.

Asked why he thought he was being targeted, Mr Berezovsky replied: "They are trying to reach me because I concentrate a group of people who create real opposition, opposition able to act, and I have enough money to support this opposition."

Earlier, Yuri Fedotov, Russia's ambassador to London, said the alleged plot to assassinate Mr Berezovsky was "quite strange information" and that he had "nothing that could confirm it".

However, he pointed out the dissident businessman was linked "to many criminal international schemes of money laundering, corruption and organised crime".

Mr Berezovsky is sought by Russia for trial on embezzlement and money-laundering charges but Britain has refused to extradite him.

In a separate development, the government insisted it would not accept a Lockerbie-style trial in a third country for Mr Lugovoi, having earlier hinted that it might.

Yesterday morning, No 10 said it wanted a trial in a British court. Later, however, Prime Minister Gordon Brown's spokesman stressed: "We want the trial to be in a British court on British soil."

This was confirmed by Jim Murphy, the Europe Minister, who, appearing before the Commons Foreign Affairs Committee, also stressed Britain had "no ambition to go wider" in its diplomatic spat with Russia, which was regarded as "a strong ally".

The EU has boosted Britain's diplomatic arm by urging Russia to "co-operate constructively" on the matter.