The EU’s top security expert has warned that up to 50,000 militant Islamists live in Europe.
Gilles de Kerchove, the organisation’s anti-terror chief, told Spain’s El Mundo newspaper that 35,000 of those are in the UK alone, prompting fears of further violence after a wave of recent terrorist attacks.
Out of those radicals living in the UK, 3,000 were “worrying” for MI5 but only 500 were under constant attention by the authorities, he said.
“We must select those who are really worrying and the most dangerous, and they should be monitored 24 hours a day, seven days a week,” he said.
Limited resources mean only a fraction of the hundreds of suspected Islamist extremists at large can be subject to intensive 24/7 surveillance at any one time.
It comes as security sources told Sky News that the threat to the UK from homegrown extremists is much more significant than that posed by returning ISIL jihadists.
Police and intelligence agencies initially feared that, as the terror group lost influence in the Middle East, extremists would return to Britain and continue the attacks at home.
In fact, far fewer jihadists returned than expected, partly because of fear of prosecution and partly because they have been killed in the fighting.
However, the threat from homegrown extremists has shot up, with self-starters and 'lone wolves’ being encouraged to carry out ‘low-tech attacks’ with their own cars and weapons.
One source said: “The threat picture has diversified enormously in recent months and is likely to remain severe for the foreseeable future.
“As [ISIL] comes under pressure in Raqqa, as they did in Mosul, the impetus to go and join them has started to dissipate, but their message remains potent for those willing to listen.
“Part of the problem is that propaganda had democratised the threat so that self-starters and lone actors can view material that is pumped out encouraging them to get out with knives or vehicles to launch these low-tech attacks.”
The warnings follow a spate of recent terrorist attacks in Britain, including at Buckingham Palace, Westminster, London Bridge and Manchester Arena.
It later emerged that the Manchester suicide bomber Salman Abedi was a “former subject of interest” to the security services whose risk “remained subject to review”, a revelation that brought the UK’s anti-terror efforts under fresh scrutiny.
Mr de Kerchove also warned that ISIL may be planning to mount deadly cyber-attacks on Britain’s critical infrastructure in the next five years, including air traffic control systems or nuclear power stations.
Based on his estimate of the numbers of terrorists, the threat now facing the UK is much graver than that in any other European country.
“The United Kingdom has identified 20,000 to 35,000 radicals,” he said.
“France has 17,000. Spain many less, but more than 5,000 I suppose. In Belgium almost 500 have been to Syria and there are around 2,000 radicals or more.”
He added that it was of utmost importance that European countries continue to cooperate on crime and security.
The issue of data and intelligence sharing has been a bone of contention in recent months, as British prime minister Theresa May was accused of treating security as a “bargaining chip” in Brexit negotiations with Brussels.
In her letter notifying the EU of the UK’s intention to withdraw from the bloc, Mrs May warned that failure to reach agreement in the talks would result in weakened cooperation in the fight against crime and terrorism.
The European parliament’s Brexit coordinator, Guy Verhofstadt, retaliated by saying MEPs would push back against any attempt by the UK to “blackmail” the EU by dangling its strength in the military and intelligence fields as a bargaining chip, underlining the complexities that the prime minister will face in achieving a smooth exit from the EU.