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.