The head of MI5 has warned that young British men are heading to new terror training grounds in North Africa and the Middle East.
Arab Spring revolutions in Libya, Syria and Egypt have created unstable areas where al Qaeda can thrive, said Jonathan Evans, director-general of the security service.
The terror group is establishing itself in those countries after being disrupted in Afghanistan and Pakistan, he said.
"A small number of British would-be jihadis are also making their way to Arab countries to seek training and opportunities for militant activity, as they do in Somalia and Yemen.
"Some will return to the UK and pose a threat here. This is a new and worrying development and could get worse as events unfold."
Mr Evans' warning came in a speech entitled 'The Olympics and Beyond', the inaugural Lord Mayor's defence and security lecture in the City of London.
He has cancelled all leave for his staff in the run-up to the London Olympics and made the Games security operation a priority.
He said: "The Games present an attractive target for our enemies and they will be at the centre of the world's attention in a month or so.
"No doubt some terrorist networks have thought about whether they could pull off an attack.
"But the Games are not an easy target and the fact that we have disrrupted multiple terrorist plots here and abroad in recent years demonstrates that the UK as a whole is not an easy target for terrorism."
He warned that the death of Osama bin Laden and the disruption of al Qaeda did not mean the British terror threat had evaporated.
He said: "In back rooms and in cars and on the streets of this country there is no shortage of individuals talking about wanting to mount terrorist attacks here."
There have been 43 potential terror plots or serious incidents in the UK since the 9/11 attacks in the United States, according to the Royal United Services Institute (Rusi) think tank.
The MI5 chief also highlighted the extent of the new threat to cyber security to business and government.
He said: "Vulnerabilities in the internet are being exploited aggressively not just by criminals, but also by states.
"And the extent of what is going on is astonishing - with industrial-scale processes involving many thousands of people lying behind both state-sponsored cyber espionage and organised cybe crime."
Mr Evans also warned there were concerns over developments in Iran and the uncertainty over its nuclear intentions.
Recent terror attacks on its enemies' interests abroad raised the spectre of a similar threat to the UK, he said.