"British nationals have a right to come to the UK", the head of MI6 has said following IS school girl Shamima Begum's plea to come home
Speaking in Munich at a security conference, Alex Younger added that returning British fighters and IS brides could potentially be "very dangerous" to the the UK.
He said: "We are very concerned about this, because all experience tells us that once someone had been put in that sort of position, or put themselves in that sort of position, they are likely to have acquired both the skills and connections that make them potentially very dangerous.
"And also of course experienced extreme radicalisation, either in their journey to that place or when they are there."
While he refused to comment on specific circumstances he said "anyone who has put themselves in that situation can expect to be investigated and questioned and potentially prosecuted if they return to our jurisdiction."
Speaking about Begum's case, Labour’s Shadow Home Secretary Diane Abbott MP said: "We are not in favour of making people stateless, that’s a punishment without due process."
She added: "If there are reasonable grounds to suspect that anyone who is entitled to return to this country either committed or facilitated acts of terrorism, they should be fully investigated and where appropriate prosecuted."
“Our priority must always be public safety. We need proper safeguards particularly if the individual is associated with an international terrorist group. All significant risks must be thoroughly assessed.”'
Sajid Javid warned he "will not hesitate" to prevent the return of Britons who travelled to join so-called Islamic State as debate flared over what should happen to runaway schoolgirl Shamima Begum.
The home secretary said those who left the UK to join IS were "full of hate for our country", while the Security Minister, Ben Wallace, warned that runaways who now want to come back must realise that "actions have consequences".
Ms Begum's family has pleaded for the 19-year-old, who is heavily pregnant, to be shown mercy and to be allowed to return to east London.
Speaking to The Times at a refugee camp in northern Syria, Ms Begum said she would "do anything required just to be able to come home and live quietly with my child”.
Her case has received high-profile backing, with a former MI6 chief saying the teenager should be given a chance "if we are to stand by our values".
Meanwhile Anthony Loyd, The Times correspondent who found Ms Begum, said she was a "15-year-old schoolgirl who made a terrible mistake… and we must do out best to rehabilitate her amongst our own people".
But her plea has been strongly rejected by others – including the brother of Alan Henning, the British aid worker beheaded by Jihadi John, who said she should "absolutely not" be allowed back.
Speaking to The Times, Mr Javid said: "We must remember that those who left Britain to join Daesh were full of hate for our country.
"My message is clear — if you have supported terrorist organisations abroad I will not hesitate to prevent your return. If you do manage to return you should be ready to be questioned, investigated and potentially prosecuted."
Any hopes of a rescue mission by British officials were swiftly quashed on Thursday as the Government ruled out an effort inside Syria to assist Ms Begum.
While refusing to comment on individual cases, Mr Wallace told the BBC: "I'm not putting at risk British people's lives to go and look for terrorists or former terrorists in a failed state.
"There's consular services elsewhere in the region and the strong message this government has given for many years is that actions have consequences."
While no official operation to remove Ms Begum from Syria will be carried out, questions have been raised over whether Britain would be able to prevent Ms Begum’s eventual return to the UK.
The former independent reviewer of terrorism legislation, Lord Carlile, told the BBC that if Ms Begum has not gained a second citizenship of another country she will have to be allowed back to her homeland because under international law it is not possible for a person to be made "stateless".
Meanwhile, Richard Barrett, a former director of global counter-terrorism at MI6, suggested it would be “unreasonable” to expect the Syrian Defence Force to look after her indefinitely.
He also warned that summary execution is the “most likely outcome” for such captured foreign nationals who are handed over to Syrian or Iraqi authorities.
Writing for The Guardian, he said: "Governments have a responsibility to address the problems created by their captured nationals and also to look more closely at why they made the choices they did."
He added: "Even (Ms Begum), as unrepentant as she may be, should be given a chance, if we are to stand by our values – and if we believe our society is strong enough to reabsorb a 15-year-old who went badly off the rails."
Ms Begum's admission that she did not regret travelling to IS-controlled Syria, and her assertion that she is "not the same silly little 15-year-old schoolgirl who ran away from Bethnal Green four years ago" has been highlighted as a cause for concern by some.
Dr Kim Howells, a former counter-terrorism minister, told the Daily Mail: "She sounds to be completely unrepentant, she sounds cynical."
Mr Henning's brother, Reg, told The Sun: "The authorities should take her passport off her. She made her choice, didn’t she? She made her bed and she should lie in it."
However, Ms Begum’s brother-in-law Mohammed Rehman told Mail Online that her family want her to be allowed to return and be "re-educated".
He said: "I can understand why people in this country are angry and don’t want her back. What she’s done doesn’t portray Islam in a good light. But she was only 15 when she went to Syria. We are appealing for compassion and understanding on her behalf."
Ms Begum was one of three schoolgirls, along with Kadiza Sultana and Amira Abase, from Bethnal Green Academy who left the UK in February 2015.
Another girl, Sharmeena Begum, also from Bethnal Green but not related to Shamima, had travelled to Syria two months earlier.
Ms Sultana was reported to have been killed in an air strike in 2016.
Shamima Begum said she had recently heard second-hand that the other two girls may still be alive.