Britain is home to up to 35,000 fanatical Islamists of whom 3,000 are 'worrying' for the security service MI5, Europe's top anti-terrorism official said today.

EU counter-terrorism coordinator Gilles de Kerchove singled out the UK as having more radicalised Muslims than any other country in Europe.

Mr de Kerchove also warned Islamic State will attempt a cyber-attack on nuclear power stations or air traffic control systems within five years.

This map shows the number of Islamist fanatics in four European countries, according to figures supplied by EU counter-terrorism coordinator Gilles de Kerchove

De Kerchove singled out the UK as having more radicalised Muslims than any other country in Europe. Pictured: Armed Police at Westminster Abbey guarding a memorial for the victims of the Westminster Bridge terror attack

This could be done by paying Russian hackers to break into vital computer systems.

The official, who is in regular contact with security services, terrorism experts and governments around Europe, said: '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, who has been in his position since 2007, was speaking in an interview with Spain's El Mundo newspaper.

He warned European countries will suffer further attacks inspired by ISIS - and said greater measures must be taken to prevent them.

Police in Catalonia had been warned about Abdelbaki Es Satty, the alleged leader of the Barcelona cell, by counterparts in Belgium

Asked about the 12-man cell who murdered 16 and injured more than 100 in attacks in Barcelona and Cambrils on August 17 and 18, he said: 'It's early to draw definitive conclusions, but we know that we need much greater efforts in prevention.

'How do we detect the first signals (of radicalisation), no matter how weak they are? They succeeded in preparing the attacks under the radar.

'We will suffer more attacks. Most, except for Brussels and Paris, were not directed from Raqqa, but they were inspired there. And later Islamic State claims responsibility.

'The propaganda of the organisation no longer requires people to go to the caliphate. They can attack in their places or origin, including on a small scale with home-made weapons.'

Mr de Kerchove said it was vital that European intelligence agencies and police properly analyse the data they collect and share.

For example, police in Catalonia had been warned about Abdelbaki Es Satty, the alleged leader of the Barcelona cell, by counterparts in Belgium, but failed to recognise the danger he represented.

The counter-terrorism official warned that radicals were increasingly using the Islamic concept of 'taqiyya' - concealing their religious beliefs from those around them - in order to prevent detection.

Mr de Kerchove said European security services had 'dismantled cells, destroyed plans, arrested people'

And he said: 'We must work more at protecting so-called easy targets. Bollards, the re-designing of pedestrian streets are necessary. There is a clear effect of imitation in terrorism, and it seems clear that something like Barcelona will happen again.'

Mr de Kerchove said European security services had 'dismantled cells, destroyed plans, arrested people. Spain has arrested 51 jihadis so far this year. In 2016, 69, in 2015, 75.'

He added: 'This is a generational problem. It will take time, decades, it won't be resolved in months.'

The official said to date there have been no recorded cases of cyberterrorism, 'in the sense of penetrating or intending to penetrate the systems of nuclear power stations, prisons, electrical stations or aerospace'.

But he added: 'I would not be surprised if in the next five years it happens. Not that it works but that they try. One should not underestimate the potential of Islamic State.

'At the moment they don't have the knowledge for this. But in the dark net you can find system vulnerabilities for sale.

'Or they can buy the services of Russian hackers, because they have money.'