Experts have warned that a terrorist attack by jihadists returning radicalised from the war in Syria is "inevitable".
It is believed that hundreds of British citizens have fought in the three-year Syrian conflict, with many taking up arms for Islamist groups.
Some have posted videos online, boasting of their exploits.
There are growing fears among security services that returning fighters with combat experience are poised to launch terrorist attacks in the UK. The Telegraph reports that around 250 Syria veterans may be back in the country.
Ahead of a discussion on Syrian fighters at Chatham House this week, Raffaello Pantucci, senior research fellow at the Royal United Services Institute, spoke of the growing danger.
"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," he told the Independent on Sunday.
Last year, security forces are reported to have foiled a Mumbai-style plot, involving killing civilians with guns in central London, which involved Syria veterans.
Richard Barrett, former head of counter-terrorism at MI6, told the paper that number of returning fighters was "almost impossible to monitor", and that some were likely to return "radicalised and a real danger to society."
The warnings come after Gilles de Kerchove, the EU's counter terrorism co-ordinator, said that European countries were failing to divert sufficient resources to deal with the problem.
In a letter to the Home Affairs Select Committee inquiry on counter terrorism, he stated that the threat from Syria veterans was "unprecedented".
"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."
He is calling for "concerted and co-ordinated action" by European countries to "avoid destabilisation … and the establishment of terrorist safe havens".
He said that intelligence sharing was vital as "it is not inconceivable that a foreign fighter could return to his home country with the intention of joining former comrades for an attack in another".