Violent groups who clashed with police in Greece and Spain over austerity cuts could launch an attack on the summer Games to ensure worldwide publicity, it is feared.
The Italian group, the Informal Anarchist Federation, has already threatened to wage a “low-intensity war” against the Olympics.
That group, known by its Italian initials FAI, has also been linked with a failed mail bomb attack on a German bank.
The hacking group Anonymous has also warned it intends to target the global event.
One security expert last night said anarchist groups on the Continent are increasingly supporting one another and may team up with UK counterparts this summer.
Valentina Soria, a counter-terrorism research analyst at the Royal United Services Institute (Rusi), warned they will look to provoke the police and the situation could “turn nasty”.
Although there is no specific intelligence of a planned attack, police and security officials have been in contact with their counterparts in the countries to monitor the situation.
That information is being fed in to the Olympic Intelligence Centre which is assessing all the potential risks and terror threats to the Games.
A senior Whitehall source told The Daily Telegraph police and security services were aware there was a risk that anarchist groups caught up in the volatile situation in southern Europe could look to target the UK when the world was watching.
It is another example of the wide range of potential threats to the Games over and above terrorism.
Groups in Greece have regularly clashed with police as demonstrations over cuts and the euro bail-out boiled over in to violence.
In March, demonstrators in Barcelona fought with police, who reacted with tear gas and baton charges.
And last month, a group claiming to be an arm of FAI sabotaged railway signal cables in Bristol.
The attack severely disrupted rail services in and out of the city and experts fear similar tactics could be deployed in London and other Olympic cities.
But there is also a risk of more serious attacks. Last year, the FAI claimed responsibility for a failed mail bomb sent to the chief executive officer of Deutsche Bank.
It was also behind a drive-by gun attack last month in Genoa.
In a statement on the anarchist website 325.nostate, the FAI said: “In the United Kingdom of clockwork control and domestication, we’re some of the “unpatriotic ones” who find the 2012 Olympics, with the ensuing spectacle of wealth … frankly offensive.
“We have no inhibition to use guerrilla activity to hurt the national image and paralyse the economy however we can. Because simply, we don’t want rich tourists – we want civil war.”
Miss Soria said Italian groups have recently come out in support of their Greek counterparts, in particular targeting German banks in protest over the Euro bail-out conditions imposed on Greece, which is effectively being driven by Germany.
She added: “We have seen the anarchists in Italy supporting the Greek anarchists more recently and they may join anarchists already in the UK for some kind of disruption of the Olympics.
“They could target Olympic sponsors. Many of them are the kind of big corporations that anarchists are more likely to target.
“At the end of the day what they want is to provoke the police and get a reaction and that could turn the situation nasty and possibly violent.”
She said of the Olympics: “It’s a global platform, a global stage that attracts people from all over the world.
“It brings to the UK problems and issues that may have nothing to do with the UK and from a country completely in another part of the world.”
A spokesman for Scotland Yard said: “We have planned against the four main areas of threat: terrorism, serious and organised crime, public disorder and natural hazards.
“Naturally our ongoing operation to ensure the safety and security of the Games also involves monitoring world events, assessing the impact they could have on any threats to the Games and taking any appropriate measures to mitigate those threats.”