Teenagers who “naively” travelled from Britain to join Islamic State in Syria should be given space to reintegrate into society on their return rather than be prosecuted, Britain’s anti-terror watchdog said today.

Max Hill QC warned against “losing a generation” of young men, and some women, by putting them through the courts after having being lured to go to the warzone by IS propaganda.

He spoke out a day after EU Security Commissioner Julian King said up to 8,000 foreign fighters in Syria and Iraq may return to Europe after the fall of the IS stronghold of Raqqa.

Around half of the estimated 850 Britons believed to have gone to join the extremists are now believed to be back in the UK.

Mr Hill, the independent reviewer of anti-terror legislation, told BBC radio: “The authorities have looked at them and looked at them hard and have decided that they do not justify prosecution, and really we should be looking towards reintegration and moving away from any notion that we are going to lose a generation due to this travel.

Isis recruit: Briton Jihadi John, real name Mohammed Emwazi, who was killed in an air strike in Syria in 2015 (Reuters)

“It’s not a decision that MI5 and others will have taken lightly.

“But they have left space, and I think they are right to do so, for those who travelled out of a sense of naivety, possibly with some brainwashing along the way, possibly in their mid-teens and who return in a state of utter disillusionment and we have to leave space for those individuals to be diverted away from the criminal courts.”

The stance faced immediate questions given that supporting a terrorist organisation such as Islamic State is a criminal offence in the UK.

Conservative MP Andrew Rosindell, who sits on the Commons foreign affairs committee, said: “The presumption should be that all people who went to join IS should be dealt with by the criminal justice system unless there is a good reason why not.

“IS is an utterly barbaric organisation still intent on carrying out terror attacks in Britain and everything possible must be done to stop its supporters being involved in such atrocities.”

Tory MP Geoffrey Clifton-Brown added: “We should take a tough approach on this. Protecting against any future terror attack must be the primary concern.”

Mr Hill stressed that he was not saying that all Britons who have joined IS should escape prosecution.

He said: “Some of those who travelled will have committed the most serious criminal offences and in any case where there is evidence of that, then there is only one place to go and that is to a criminal court in this country to decide on guilt or innocence.”

Richard Barrett, former counter-terrorism director at MI6, added: “Many of them went to join something new, something that looked bright and attractive and satisfied some of the needs in their lives and probably found that did not exist out there and so came back highly disillusioned.

“But also somebody going off to join the Islamic State is not likely initially to be somebody going off to train to be a domestic terrorist.”

MI5 chief Andrew Parker warned earlier this week that the terrorist threat facing Britain was now worse than at any time in his 34-year career.

He said MI5 has more than 500 live investigations, with 3,000 people suspected of being involved in extremist activities.