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Said two in five of Europol terrorism cases

Many of these now returning to Europe potentially

The threat of terrorist attacks in the EU is at its highest level in ten years, as hundreds of European ISIS fighters get ready to return to their home countries, the head of Europol has warned.

Rob Wainwright said at least 5,000 Europeans have become radicalised and left to go to Syria and fight for ISIS, with many now returning, potentially planning terrorist attacks across the continent.

The Europol director added that nearly half of all terrorism cases handled by the European Union's law enforcement agency involved a 'British element'.

High risk: Rob Wainwright, the director of Europol, said that nearly half of all terrorist cases handled by the European Union's law enforcement agency involved a 'British element'.

The former MI5 officer said that the chance of a terror attack on the scale of Paris was growing and European co-operation was needed.

'I think it is likely that we will have another attack,' Mr Wainwright told Sky News.

'We are working of course around the clock to prevent that from happening but this is a very, very serious threat.'

Mr Wainwright said the current threat level was the highest it had been for ten years and that the counter-terrorism community needed to up its game to protect citizens.

Returning: Wainwright said at least 5,000 Europeans have become radicalised and left to go to Syria and fight for ISIS with many now returning, potentially planning terrorist attacks across the continent

Tragedy: The terrorist attacks in Paris on November 13, where 130 people were killed and 352 people were injured, a majority of the attackers were French and Belgian nationals radicalised by ISIS.

Commenting on the upcoming referendum on EU membership, Mr Wainwright warned that a Brexit would have an impact on the UK's security capabilities.

Up to 40 per cent of Europol's casework involved a 'British element', Mr Wainwright said.

He said: 'One can expect that the arrangements will be not as optimal (in the event of the UK leaving the EU) and therefore it will be difficult for the UK to replace what it currently relies on in the EU with arrangements that would be as effective and as cost efficient as well.'

This comes just two days after a warning by the UK's top anti-terror officer that ISIS is planning a 'spectacular' attack on British soil.

Warning: Scotland Yard Assistant Commissioner Mark Rowley, pictured, said Mondat that ISIS is 'trying to build bigger attacks' in Europe - and has its eye on the UK

Scotland Yard Assistant Commissioner Mark Rowley has said the terror group is 'trying to build bigger attacks' around the world and the UK is one of its top targets.

The Met officer said that while in the past few years the Islamist group has called on would-be jihadis to attack police and the military, their plots are now broader 'plans to attack Western lifestyle'.

Mr Rowley, who is the national policing lead for counter-terrorism, said that ISIS is trying to get supporters who have received military training in Syria into northern Europe to stage attacks.