Royal Marines take part in security exercise on Thames in preparation for Olympic Games (Reuters)

Heads of state attending the London 2012 Olympic Games face the threat of assassination, according to a security thinktank.

A report by the Royal United Services Institute analyses security threats to the games.

"Some 120 heads of state will attend the games and many have enemies who may have links or even live in the UK, but are not followed up here because they are no threat to UK security," according to the report, Counter-Terrorism in an Olympic Year.

"In effect, the Olympics may attract someone else's terrorist problem to the UK because of the opportunities and publicity the games afford," the report said.

As an example, it cites the 1972 Munich Games, when 11 Israeli athletes were killed be Palestinian terrorists.

Heads of state and government from all the competing countries will be invited to attend London 2012's opening ceremony on 27 July.

IRA, al-Qaida, and Right-Wing Extremist Threats

Other terror threats to the games include dissident Irish republicans and Islamic extremists.

The report warns that a lone wolf-style attack, in which one person goes on a rampage, represents the "biggest threat".

Although such a threat is more likely to come from an Islamic extremist, it "could extend to non-jihadist terrorism, as we saw in the case of the Norwegian bomber Anders Behring Breivik," the report said.

A group of MPs recently warned of the rising terror threat from violent right-wing extremists like Breivik, who killed 77 people in twin bomb and gun attacks in Norway in July.

Dissident Irish republicans also yearn to make a fresh attack on English soil.

"Since their last successful bombings in England in 2001, police sources in Northern Ireland believe they have aspired to attack again, but either have not managed to do so or have chosen not to," the report said.

While they are not seen as a threat on the same level as jihadists, they cannot be ruled out and remain a danger, it concluded.

Games' Security Questioned

Fears about security were intensified after undercover police managed to smuggle a fake bomb into the Olympic Park in Stratford, east London, under the noses of security officers and sniffer dogs.

It was part of a routine exercise to test the security arrangements.

One senior police officer working on the games also mistakenly left secret documents detailing security arrangements for London 2012 on a train.

Police insisted that security had not been breached.