He sought to shoot exiled tycoon Boris Berezovsky — who has called for the violent overthrow of Russian president Vladimir Putin — in the back of the head.
The assassin was accompanied by a CHILD in a cold-blooded attempt to avoid raising suspicion.
But MI5 and MI6 intercepted intelligence about the plot — due to have been carried out within the last fortnight.
And the hitman was seized before he could open fire.
The murderous mission was revealed 24 hours after Britain ordered the expulsion of four Russian diplomats.
And its disclosure will plunge the cold relations between London and Moscow into the deep freeze.
Mr Berezovsky, 61, fled to Britain from Russia in 2000 and was granted political asylum three years later.
Today he mixes in the highest echelons of society and lives in a 172-acre Surrey estate he bought for £10million from radio DJ Chris Evans.
A soccer nut, he has a box at Arsenal’s Emirates stadium and was a friend and business partner of Chelsea owner Roman Abramovich before the pair had a spectacular falling-out in 2005.
Mr Berezovsky was also a pal of former KGB spy Alexander Litvinenko — poisoned with a lethal dose of radioactive Polonium-210 in a London sushi bar last November.
The targeted tycoon is at the centre of a hotbed of opposition to the Putin regime — and is the Russian president’s fiercest critic.
He is understood to have offered cash, housing and support to a string of Russian exiles, including Litvinenko.
The hitman planned to strike after luring Mr Berezovsky to a meeting in a room at the Hilton.
But details of the plot were rumbled by Britain’s security services.
Together with anti-terrorist cops from Scotland Yard, they mounted a round-the-clock surveillance operation to shadow Mr Berezovsky and the assassin.
They took over a room adjoining the meeting place and seized the hitman before he could open fire.
A source said: “The Russian suspect was monitored attempting to buy obvious weaponry for the planned mission.
“Disturbingly, he was accompanied by a child in an attempt to blend into the background. This ‘family persona’ tactic is also thought to have been used in the murder of Litvinenko.”
Security officials stressed there was no direct connection between the assassination plot and the row which has led to the expulsion of the Russian diplomats. The officials got the boot after Mr Putin refused to allow the extradition from Moscow of former KGB agent Andrei Lugovoy, the prime suspect in the Litvinenko case.
But despite the lack of a link, a senior Government security source said: “We cannot tolerate a situation where Russian hit squads can roam the streets of London trying to take out enemies of their regime.
“In the case of Litvinenko, the lives of hundreds of Londoners were put at risk by the use of a radioactive substance.
“It is clear Mr Berezovsky has been a persistent critic of Mr Putin and his regime. It is our experience that the Russians have no compunction about taking action against their critics abroad.
“We only have to look at the cold-blooded murder of Mr Litvinenko to see the lengths to which they will go.”
Last night Mr Berezovsky said there had been a series of plots to murder him in Britain.
He told the BBC’s Newsnight: “There were several attempts to kill me in this country.”
He added: “Scotland Yard pay a lot of attention to my protection and I’m happy about that.”
He also accused Mr Putin of killing Mr Litvinenko. He said: “I’m 100 per cent sure that behind this murder is not just Andrei Lugovoy but Putin himself. That’s the reason Russia protects Lugovoy, because they are protecting Putin.”
Mr Berezovsky built a fortune after the Cold War ended in car sales, oil and the media.
His estate is guarded by a squad of ex-French foreign legionnaires. The complex features bullet-proof windows, reinforced steel doors, laser monitors and spy cameras.
Even before the Putin era, he was in the sights of the Russian mafia, surviving several attempts to kill him.
In 1994 his Mercedes was blown up by a car bomb, decapitating his chauffeur and leaving Berezovsky with serious burns.
When Putin came to power, he was at first a cordial friend of the new President. But they fell out over Putin’s decision to probe how oligarchs like Mr Berezovsky made their money. The tycoon fled to Britain when charges of tax evasion and embezzlement were brought against him.
He now runs several businesses here while also plotting Putin’s demise. He boasts of bankrolling a group in Russia who aim to bring Putin down by force.
Last night security expert Chris Dobson said the plot to assassinate him showed “Putin is behaving like the thug he always was”.
Mr Dobson added: “The Russians have always believed it is their right to assassinate their enemies wherever they are. They take no notice of diplomatic treaties.”
A Scotland Yard spokesman said: “We cannot make any comment at this stage.”