Britain is home to 35,000 Islamist fanatics, more than any other country in Europe, the EU anti-terror chief has said.
Gilles de Kerchove warned that of those, 3,000 were “’worrying” for MI5.
He also warned Islamic State will attempt a cyber-attack on nuclear power stations or air traffic control systems within five years, potentially by paying paying Russian hackers to break into vital computer systems.
Mr de Kerchove told Spain’s El Mundo newspaper: “The United Kingdom has identified 20,000 to 35,000 radicals.
“Of these, 3,000 are worrying for MI5, and of those 500 are under constant and special attention.
“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.
“I wouldn’t like to put a concrete figure on it, but (in Europe) tens of thousands, more than 50,000.
“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.”
Mr de Kerchove said it was vital that European intelligence agencies and police properly analyse and share the data they collect.
Figures published in May suggested that a further 20,000 individuals had been considered “subjects of interest” in the past, although the period the figures cover is unclear.
Anti-terror efforts came under fresh scrutiny following revelations that suicide bomber Salman Abedi had been a “former subject of interest” to MI5 who was “subject to review”.
Security minister Ben Wallace said at the time: “All those people are in the mix and they have to be looked at.”
A senior Whitehall source said 18 plots had been foiled since 2013 in Britain, including five since the Westminster atrocity in March this year.
Round-the-clock monitoring of terror suspects is only possible in a handful of cases due to limited resources, the priority of which are determined on factors including credibility of intelligence and the apparent seriousness of the individual’s intent.