Islamic extremists, independent of al-Qaeda, may be the most likely source of such an attack but dissident Irish republican groups should not be discounted, according to a briefing seen by Metro.
A £1billion security lockdown to protect the Games involves surface-to-air missiles and military aircraft.
However, risks remain high on several fronts, according to a report due to be published tomorrow.
Will Hartley, of defence specialists IHS Jane’s, said Breivik showed the impact ‘lone wolves’ can have.
He added: ‘By definition, they are unpredictable and could be potentially motivated by any number of causes.’
So-called ‘chatter’ discussing the targeting of the Games was evident on militant Islamist internet forums in recent months, Mr Hartley warned.
Anarchists represent the most credible threat to the Olympics, however are ‘extremely unlikely to pose a deliberate threat to life’.
It is questionable dissident republican groups ‘would consider targeting international crowds’, especially having not targeted the Jubilee.
Residents yesterday lost their High Court battle to prevent surface-to-air missiles being deployed on the roof of their apartment block.
A judge ruled those living in the Fred Wigg Tower, east London, did not have an arguable case.