“The retaking of the IS’ northern Iraq stronghold, Mosul, may lead to the return to Europe of violent IS-fighters,” the EU Commissioner for security Julian King said in an interview with German dialy Die Welt.

The British commissioner, who was named last August, said the European Union should be prepared for returning jihadists who would pose a “serious threat that we must prepare ourselves for”.

Some 30,000 Iraqi forces are leading the offensive against IS, backed by air and ground support from a 60-nation US-led coalition.

The British European Commissioner was named last August.

Gilles de Kerchove, the EU’s counterterrorism coordinator, told Newsweek last week that there are concerns that returners will have acquired new and dangerous skills in their time in ISIS’s ranks. “One of the risks is that these guys have learned a lot in terms of weaponized chemical stuff, in terms of using car bombs,” he said. “I hope this knowledge will not be used here, but we have to be vigilant.”

Europol, the EU’s law enforcement agency, estimates that at least 5,000 Europeans have traveled to Syria and Iraq to fight for jihadi groups but only a third of them have returned.