Home Secretary Amber Rudd said: “It is absolutely a serious threat.
Families coming back will be potentially having children who are going to be vulnerable, who are going to need protecting; but also potentially fighters themselves who could be a danger to society and could radicalise other people.”
Ms Rudd was giving evidence at the Intelligence and Security Committee about threats to the UK. At least nine terror plots have been foiled this year.
Spies told the committee more than 850 Britons of “national security concern” had gone to Syria, Iraq and the region.
MI5 said: “Around half of those have returned to the UK, more than 300 are thought to remain in Syria, and around 100 have been killed in fighting.”
ISC chairman Dominic Grieve said: “The scale of the terrorist threat facing the UK is unprecedented... the dispersal of foreign fighters from Syria and Iraq raises serious questions as to when and where they will resurface, and with what intent.”