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.