Britons are increasingly heading to the Middle East for terrorist training in the wake of the Arab Spring, the head of MI5 has said.
Parts of the Arab world are becoming a more permissive environment for al Qaeda warned Jonathan Evans, the director-general of the Security Service, and Britons are travelling there to "seek training and opportunities for militant activity".
Some will return to the UK and pose a threat and the situation "could get worse as events unfold", he said.
In a rare public speech just a month before the Olympic Games begin in London, Mr Evans said the Arab world was in "radical transition".
It is believed 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.
MI5 chief Jonathan Evans gave a rare speech last night, ahead of the Olympics
"Today parts of the Arab world have once more become a permissive environment for al Qaeda," 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.
"So we will have to manage the short-term risks if there is to be a longer-term reward from the Arab Spring."
Yemen, Libya, Nigeria and Egypt are all understood to pose a risk.
However, foreign secretary William Hague urged the public "not to lose faith" in the Arab Spring via Twitter, adding that "positive change takes years to entrench."
Speaking at Mansion House in central London, his first public speech since September 2010, Mr Evans said Britain had experienced a "credible terrorist attack plot about once a year since 9/11".
Despite the perception in some quarters that, with terror mastermind Osama bin Laden dead and al Qaeda'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.
"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," he added.
"The threat is real and remains with us today.
"But we do see a changing shape of the threat internationally.
"We appear to be moving from a period of a deep and focused threat to one where the threat is less monolithic but wider."
Mr Evans also warned about the astonishing scale of cyber attacks, "with industrial-scale processes involving many thousands of people lying behind both state-sponsored cyber espionage and organised cyber crime".
Evans also spoke about the threat of cyber attacks
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.
MI5 is also investigating potential attacks in more than a dozen other companies, as well as working with others who may be targets in the future.
He said: "So far, established terrorist groups have not posed a significant threat in this medium, but they are aware of the potential to use cyber vulnerabilities to attack critical infrastructure and I would expect them to gain more capability to do so in future."
He urged company boards to consider the vulnerabilities of their own companies.
Mr Evans added that the economic crisis in the eurozone could also lead to the rise of political extremism, but noted that the British have been "stolidly unimpressed" by this in the past.
"I suspect that any problems we may have here will come from lone actors attracted to extremism and violence rather than an organised political movement," he said.
But last year's attacks in Norway showed "how devastating a single individual can be if sufficiently determined and callous", he said.