A growing number of Britons are heading to the Middle East for terrorist training, in preparation for attacks in the wake of the Arab Spring, the head of MI5 has said.
Jonathan Evans, the director-general of the Security Service, warned that parts of the Arab world were becoming a more permissive environment for al Qaida and Britons were travelling there to "seek training and opportunities for militant activity".
It is estimated that up to 200 British residents and nationals are currently in the Arab world and either involved in training camps, being radicalised or operationally active with terror groups.
In a rare public speech at Mansion House, just a month before the Olympic Games begin in London, Mr Evans said the Arab world was in "radical transition".
"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."
The perception in some quarters that, with terror mastermind Osama bin Laden dead and al Qaida's senior leadership under serious pressure in Pakistan, the terrorist threat to the UK has evaporated, in reality it will outlast the Olympics, he said.
Mr Evans also warned about the scale of cyber attacks, "with industrial-scale processes involving many thousands of people lying behind both state-sponsored cyber espionage and organised cyber crime".
One firm listed on the London Stock Exchange estimates it has lost some £800 million as a result of a hostile state-sponsored cyber attack, he said.