THE HAGUE (AFP) - Europe may have to set up new ways of sharing vital intelligence to protect itself in the fight against terror groups if Britain leaves the EU, the top Dutch anti-terror chief warned on Wednesday.

Asked if Thursday's vote could impact the security of Europe, national counter-terrorism coordinator Dick Schoof replied "of course" but that could be the case even without a Brexit.

"If you don't get information from a country... there is a big danger for other countries," Schoof told foreign reporters.

Intelligence cooperation with Britain, which has a large number of foreign fighters who have joined the so-called Islamic State in Iraq and Syria (ISIS) group, is "excellent," Schoof said.

But in the event of Britain leaving the European Union, "we would have to find new arrangements. It's more difficult, but we would have to find a way."

The 28-members of the European Union already share tip-offs and information, particularly through the European police agency, known as Europol.

But if Britain votes to leave the bloc, it would cease at some point to be a member of Europol.

Schoof stressed "we can always work around any problem" created by Britain's departure "because I think that every country is absolutely convinced that sharing information is key to the work of law enforcement and intelligence."

In the wake of the Paris attacks in November and the March assault on the Brussels metro and airport, Schoof warned the terror level posed by Islamic militant fighters against European targets "is high, remains high and will probably remain high in the near future."

"We still have big worries that operatives from ISIS (IS) are in Europe and are trying to get a network together again and attack."

"The threat level to the Netherlands is substantial which means that an attack in the Netherlands is a real danger and can actually happen," he added.

About 250 Dutch people are believed to be fighting alongside ISIS and other terror groups in Iraq and Syria.

Despite working hard to stop radicalisation of people, "four, five, six people are still going per month" to join ISIS, he said.

But Schoof stressed the numbers could have been much higher if the Netherlands had not taken a slew of measures.

Last month, MPs voted in favour of stripping dual nationals of their Dutch citizenships if they joined terror groups - although it still has to become law.

Schoof said around 250 passports of people wanting to leave the country to join militant groups in the Middle East had also been confiscated.