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Britain was today warned of a rising threat from Islamic State fighters sent “on mission” from Syria.

The alert came as the EU’s top law enforcement officer revealed that increasing numbers of jihadis are using fake documents to sneak into Europe.

Europol’s director Rob Wainwright said IS had taken a “strategic decision” to send its fanatics to attack the continent in an attempt to distract attention from battlefield defeats in its heartland.

He also highlighted the weapons trade on the “Dark Net”, saying it was a “major part of the security challenge” that is now facing Europe. Mr Wainwright said some IS extremists were using false Syrian passports in a bid to arrive undetected — with a small but increasing number posing as refugees.

Others were exploiting the “industrial scale” production of false documents by criminals to obtain EU passports and move freely over the continent.

Mr Wainwright said Europe also faced the return of thousands more extremists as IS crumbled in Syria in a “long, long struggle” that will pose an “onerous security challenge” for years.

The warnings, in an interview with the Standard at Europol’s HQ in The Hague, came as Mr Wainwright also:

Said a new squad of 200 counter- terrorism officers is to be deployed to the Greek islands within weeks to spot extremists seeking entry to Europe.

Predicted more attempted Paris-style “spectacular” attacks, saying there are more than 50 counter-terror investigations under way in Europe.

Warned of reports that extremists are attempting to radicalise migrants at refugee reception centres in Greece and the Balkans.

Revealed that “hundreds” of law enforcement operations are trying to stop illegal firearms being moved to Britain and other EU nations.

His comments come amid reports by the Italian newspaper La Stampa that fake passports intended for use by IS extremists were found by law enforcement staff at refugee camps in Greece.

That will heighten concern about the terror threat facing Britain and follows a recent warning by Met Commissioner Sir Bernard Hogan-Howe that it is a matter of “when, not if” Britain suffers another terrorist atrocity.

In his interview today, Mr Wainwright warned that this threat by returning foreign fighters was increasing.

He said: “There’s a lot pressure on IS now. I suspect morale might be flagging and a number of fighters of their own volition will want to return. Over several years we’ll have to deal with the re- integration of thousands of Europeans who’ll come back having been exposed to a highly radicalised environment.

“That’s going to be a long, long struggle for us to deal with the numbers involved and how we can get them back into society, plus sort out which among them pose the biggest security threat.

“Given the numbers involved, that’s quite an onerous security challenge the authorities will face. Some fighters will be sent back by IS to engage in terrorist activity. We have, in the last year and a half, seen a strategic decision by IS to do that and carry out spectacular attacks of the type we saw in France and Brussels. There will be further attempts at that kind of activity, not least as a distraction.

“If they are having less military success in Syria then the ability to carry out spectacular attacks in Europe is an alternative way to sustain morale among their fighters and demonstrate that IS is still being successful. That is a strategic path IS is trying to take.

“On the security challenges we face, we’ll have a long-term struggle to reintegrate returning fighters, some of whom will have been sent back on mission.

“The other is the possibly even larger number who have never been to Syria but are still capable of being radicalised to carry out attacks.”

On the use of migration routes, Mr Wainwright said he was “surprised” IS had not exploited the refugee crisis more extensively so far, but warned that the number of foreign fighters entering Europe with false documents was growing.

“We’ve had cases, and they continue to be identified, of clandestine attempts at return — forged passports, assumed identity, typically a Syrian passport, to get into Europe, sometimes through the migratory/refugee channel.

“Some cases involve the use of a fake Syrian ID to claim asylum and on arrival in Greece — then help by an accomplice for onward journeys to other European countries.

“At this point in some cases another assumed identity is taken on, this time with a forged EU passport to travel within the Schengen free movement area. We continue to see more cases.” Mr Wainwright said that to reduce the danger Europol would deploy counter-terrorism officers to help border guards spot potential extremists. The system would be modelled on the techniques of British law enforcers at Heathrow.

He said: “The officers will, on rotation, be deployed to the Greek islands, maybe Italy. There will be a second line of defence. We hope to deploy some into the camps where the refugees, the asylum seekers, are being held.

“We are concerned over reports these kinds of reception centres are being targeted for radicalisation activities.”

At least two of those behind the Paris attacks last November are known to have re-entered Europe from Syria via Greece using false Syrian passports.

Mr Wainwright said Europol, which coordinates intelligence sharing be- tween EU members, was also trying to identify and bring to trial those trading in illegal arms and fake documents.

He added: “The criminal underworld is supplying industrial amounts of forged documents — principally for people-smuggling but some of which we know are being directed to terrorist networks. We also have tens of thousands of suspects involved in the illegal firearms trade.

“A pretty large-scale criminal underworld is supplying illegal firearms to mainly criminal contacts, a small but significant proportion of which are destined for use by terrorists.

“The supply is still connected a lot with the former Yugoslavia and a fair bit is coming from America.

“Also a lot of it is managed, arranged, bought, sold and traded online — especially on the Dark Net.

“The ability of police to identify both the buyer and seller there is very low.

“There are large-scale trading sites for illegal firearms — as there are for drugs — on the Dark Net, which is accounting for a significant part of this problem.”

On the impact of Brexit, Mr Wainwright said British law enforcement would benefit from retaining the European Arrest Warrant and access to the Schengen Information System.

Under that system, data is shared Europe-wide on people or objects with suspected connections to crime.

Mr Wainwright said an unprecedented deal would be needed to secure this but he was “optimistic” that Theresa May could negotiate a successful outcome.