Update, 9 June: Mr Wainwright has since stated he was misquoted in the original article by Neue Osnabrucker Zeitung. Europol estimates that between 3,000 and 5,000 EU nationals have fought abroad but it is not clear how many have returned to the EU. Read more here

Up to 5,000 jihadists are feared to be in Europe after returning from terrorist training camps, the head of Europol has said.

Rob Wainwright, director of the EU-wide law enforcement agency, predicted further attacks by Isis following the massacres in Paris that killed 130 people in November.

“Europe is currently facing its biggest terror threat in more than a decade,” he told Germany’s Neue Osnabrücker Zeitung newspaper.

Terror threat to UK is grave

“One can expect Islamic State (Isis) or other religious terror groups to stage an attack somewhere in Europe with the aim of achieving many casualties among the civilian population.

“Additionally, there is the risk of individual attackers – this has not reduced.”

Europol estimates that there are between 3,000 and 5,000 Europeans who have travelled to terrorist training camps abroad, including those run by Isis in its territories in Iraq and Syria, and returned home.

Several of the Paris attackers slipped back into the continent by using fake documents to pose as refugees, travelling along the well-trodden route via Turkey, Greek islands and up through the Balkans, raising fears that Isis was using the crisis to traffic its fighters.

In pictures: Paris attacks Show all 25 1 /25 In pictures: Paris attacks In pictures: Paris attacks Paris attacks French police with protective shields walk in line near the Bataclan concert hall Reuters In pictures: Paris attacks Paris attacks Rescuers evacuate an injured person on Boulevard des Filles du Calvaire AFP In pictures: Paris attacks Paris attacks French Vigipirate troops mobilize next to Place de la Bastille AFP In pictures: Paris attacks Paris attacks French soldiers mobilize near to the Place de la Bastille AFP In pictures: Paris attacks Paris attacks Wounded people are evacuated outside the scene of a hostage situation at the Bataclan theatre EPA In pictures: Paris attacks Paris attacks People react as they gather to watch the scene near the Bataclan concert hall Reuters In pictures: Paris attacks Paris attacks French police secure the area outside a cafe near the Bataclan concert hall Reuters In pictures: Paris attacks Paris attacks Rescuers workers evacuate victims near the Bataclan concert hall AFP In pictures: Paris attacks Paris attacks French Prime Minister Manuel Valls and French President Francois Hollande attending an emergency meeting at the Interior Ministry AFP In pictures: Paris attacks Paris attacks Spectators invade the pitch of the Stade de France after explosions were heard outside AP In pictures: Paris attacks Paris attacks A man lies on the ground as French police check his identity near the Bataclan concert hall Reuters In pictures: Paris attacks Paris attacks Police officers man a position close to the Bataclan theatre AFP In pictures: Paris attacks Paris attacks Wounded people are evacuated from the Stade de France in Paris EPA In pictures: Paris attacks Paris attacks Two men evacuate the Place de la Republique square in Paris as a police officer looks on AFP In pictures: Paris attacks Paris attacks Football fans are evacuated from the Stade de France stadium In pictures: Paris attacks Paris attacks An armed police officer Dan Gabriel In pictures: Paris attacks Paris attacks The Stade de France is evacuated after reports of an explosion In pictures: Paris attacks Paris attacks A member of the French fire brigade aids an injured individual near the Bataclan concert hall In pictures: Paris attacks Paris attacks Wounded people are evacuated from the Stade de France in Paris In pictures: Paris attacks Paris attacks Police are seen outside a cafe in 10th arrondissement of the French capital Paris, In pictures: Paris attacks Paris attacks Rescuers assist an injured man on Boulevard des Filles du Calvaire, close to the Bataclan concert hall AFP In pictures: Paris attacks Paris attacks The scene at a restaurant in 10th arrondissement In pictures: Paris attacks Paris attacks The Bataclan theatre - where around 100 people are thought be held hostage In pictures: Paris attacks Paris attacks The Stade de France as it was evacuated In pictures: Paris attacks Paris attacks Forensic experts inspect the site of an attack outside the Stade de France stadium in Saint-Denis AFP

"The growing number of these foreign fighters presents new challenges for EU countries,“ Mr Wainwright said.

But he confirmed that there was no concrete evidence of the group “systematically” using the refugee crisis to infiltrate the continent, adding that Europol’s new European Counter Terrorism Centre to boost intelligence co-operation.

At least 700 people from the UK have travelled to support or fight for jihadist groups in Syria and Iraq, according to police, and about half have since returned.

A senior British intelligence official said last month that it had become “virtually impossible” for security officials to detect potential terrorists among refugees as the use of fake passport continues.

The fake Syrian passport left by one of the Paris attackers at the Stade de France (AFP/Getty Images)

Isis is known to hand out counterfeit Syrian and Iraqi documents to disguise its recruits as they enter the continent, often via Turkey and Greece.

That was the route taken by at least two of the Paris attackers, including one whose fake Syrian passport was left at the scene of the Stade de France suicide bombings.

All members of the terror cell who carried out the massacres are believed to have trained with Isis at its stronghold of Raqqa before making their way into Europe.

The suspected ringleader, Abdelhamid Abaaoud, used Isis’ propaganda magazine to boast of his travels between Belgium and Syria “despite being chased after by so many intelligence agencies”, although the article did not specify the method.

A suicide bomber who killed 10 German tourists in Istanbul on 12 January had also entered Turkey from Syria as an “ordinary migrant”, the country’s Prime Minister said.

Police investigate the area after an explosion in central Istanbul

The man, carrying documents claiming he was Syrian and born in 1988, had registered with a refugee agency and provided fingerprints

Manuel Valls, the French Prime Minister, said Isis was “taking advantage” of the refugee crisis and called for the EU’s external borders to be strengthened.

The UN’s former High Commissioner for Refugees, António Guterres, has urged countries not to use the threat of terrorism to turn their back on the hundreds of thousands of asylum seekers continuing to flee conflict and persecution.

“It is not the refugee outflows that cause terrorism, it is terrorism, tyranny and war that create refugees,” he said.