More than 300 Britons are fighting with jihadist groups in Syria, raising concern that they will return trained in the latest terrorist techniques.
Syria is considered within Whitehall as a greater threat to national security than the al-Qaeda heartlands on the Afghan-Pakistan border because of the sheer numbers of British Islamists heading to the war zone.
Intelligence sources told The Telegraph that Britons make up the largest contingent out of about 1,000 Westerners fighting with Islamist groups against the regime of Bashar al-Assad.
France and Australia each have about 200 citizens fighting in Syria, with others coming from countries including the US and Canada.
On Tuesday, Richard Walton, the head of Scotland Yard’s counter-terrorism command, said that children as young as 16 were travelling to Syria to fight.
The prospect of hundreds of battle-hardened extremists returning to Britain with sophisticated training and practical experience of bomb-making and weaponry has become a grave cause of concern in recent months.
A senior Whitehall source said: “The large number of British Muslims travelling to Syria to wage jihad against the Assad regime is developing into a major security issue for the UK.
They are openly associating with Islamist terror groups like al-Qaeda, and the concern is that, once they have finished fighting in Syria, they will try to return home and wage jihad on the streets of Britain.
“Not only will they be battle-hardened as a result of their experience in Syria, they will also have been trained in all the latest terrorist techniques.”
Mr Walton, speaking at a conference organised by the business group London First, said there were already indications that Britons were returning from Syria with orders to carry out attacks, with the Metropolitan Police carrying out a “huge number of operations” to protect the public.
He said: “I don’t think the public realises the seriousness of the problem. The penny hasn’t dropped. But Syria is a game-changer. We are seeing it every day.
You have hundreds of people going to Syria, and if they don’t get killed they get radicalised. So it’s the impact when they come back.
“I think the implications over the next three to five years are very profound. We have got probably around 200 Britons who have gone to Syria and some have returned.”
Theresa May, the Home Secretary, is considering stripping terrorism suspects of their citizenship by cancelling their passports if they go abroad to fight, preventing them from returning to Britain.
Such a move would leave jihadists stateless, but Mrs May has repeatedly said that a British passport is a “privilege, not a right”.
She has revoked the British passports of 16 dual nationality terrorism suspects and plans to go further by passing a law that would allow her to do the same with people who have only one nationality.
Last month Andrew Parker, the head of MI5, told a parliamentary committee that Syria had become “a very attractive place for … those who support or sympathise with the al-Qaeda ideological message”.
Some of the jihadists are using social media websites to boast about their experiences of “five-star jihad” in the hope of encouraging others to follow them.
They include Islamists from London and Portsmouth.
By Con Coughlin, and Gordon Rayner