Thousands of foreign militants, including hundreds of Britons, are now in Syria according to government sources. They are gaining "combat experience and forging connections with extremists". As a result they could "return radicalized" and "seek to carry out attacks against the West," The Independent wrote on Sunday.

The threat posed by veterans from Syria is "unprecedented", according to Gilles de Kerchove, the EU's counter-terrorism coordinator. The warning is made in written evidence to a Home Affairs Select Committee inquiry on counter terrorism, published last Tuesday.

"All the reports I have seen suggest that it is becoming increasingly acute," he added. "National budgets devoted to counter-terrorism are declining across the EU. Yet the threat that we face is becoming more diverse, more diffuse, and more unpredictable."

Nor is the problem confined to Syria. Africa is now a "particular" concern, since "the terrorism threat is growing and becoming a major obstacle to development", de Kerchove warned.

Terror experts echoed his concerns, in the run-up to a talk on foreign militants due to be hosted by the Chatham House think-tank on Thursday.

The number of militants means they are "almost impossible to monitor", Richard Barrett, the former head of counter-terrorism at MI6, told The Independent. While many will return and not be a problem, some will come back "radicalized and a real danger to society".

He said that simply arresting people returning from Syria was a "knee-jerk reaction" and that such heavy-handed treatment by the police risked radicalizing people.

Raffaello Pantucci, senior research fellow at the Royal United Services Institute, said: "It seems almost inevitable that some sort of a threat back to the UK will come off the battlefield in Syria, something supported by the fact that security services in the UK believe they have already disrupted at least one plot with links to Syria."

In a statement, a Home Office spokesman said: "The UK advises against all travel to Syria. Even people travelling for well-intentioned humanitarian reasons are exposing themselves to serious risk, including being targeted for recruitment by terrorist groups."

He added: "People thinking about travelling to Syria to engage in terrorist activity should be in no doubt that we will take the strongest possible action to protect our national security."

NTJ/HH