Research by The Soufan Centre estimated that at least 425 British Isis members have so far returned to the UK – the largest cohort in Europe.

The figure, based on official government records, included women and children, Jeffrey Ringel, a director at The Soufan Center, told Al Jazeera.

“The concern here is the returnees may continue their fight after they return to their ancestral lands,” Ringel told Al Jazeera.