Up to 150 terror suspects stripped of their British citizenship could attempt to return to challenge those rulings if Shamima Begum’s case stands, the security services and politicians fear.
The warning follows an unprecedented court of appeal judgement on Thursday that Begum, 20, should be allowed to return to the UK to fight the Government's decision to revoke her British citizenship for joining Isil in Syria.
The three judges, led by Lord Justice Flaux, ruled that the Jihadi bride, who left the UK aged 15 to join Isil, could not have a “fair and effective” appeal unless she was present in person to give “proper instructions or provide evidence.”
They said this outweighed any risks to national security which could be “addressed and managed” if she returned to the UK.
Within minutes of the judgement, the Home Office said it would appeal the “very disappointing” ruling and seek a stay on her return until the appeal was complete.
The security services and senior politicians fear the estimated 150 terror suspects deprived of their citizenship and their lawyers will be studying the judgement to see if they could make similar appeals.
A Whitehall source said: “Any returning jihadi is a headache. Anybody who went out there to join Isil is problematic. We would have concerns about anybody who has been in her situation. Even if you didn’t think she was dangerous, you would have to switch resources to put her under surveillance.”
Sajid Javid, who as Home Secretary revoked Begum’s citizenship, said he was “deeply concerned.”
“First and most critically, allowing her - and indeed other terrorists - back into the UK to pursue an appeal would create a national security risk that cannot be fully mitigated, even with the division of significant resources,” he said.
“Second, the judgement and precedents set in this case could bind the hands of the Government in managing past and future cases.
“And third, a consequence of this individual walking our streets would be that it serves as a lightning rod for both Islamist and Far Right terrorists.”
Dr Alan Mendoza, executive director of the Henry Jackson Society, said: “The deeply troubling implication of this judgment is that up to 150 terrorists are now legally entitled to enter the UK in order to appeal the decision in their case.
“The already overworked security services will have their work cut out with this potential sudden influx. This decision could have dramatic repercussions for our entire counter-terror strategy.”
Read more: What crimes has Shamima Begum committed?
It is thought the Court of Appeal has never made such an order before in relation to suspects barred from the UK and stripped of citizenship.
Jonathan Hall, QC, the independent reviewer of terrorism legislation, told The Daily Telegraph that Begum’s personal situation appeared to have been highly influential in the case.
He said judges did not want to prejudge the issue of her citizenship but recognised how young she was when she left the UK and felt able to conclude that her national security risk was “less serious” than others and could be managed.
Mr Hall believed it could have ramifications for some among the estimated 60 British children born to Isil fighters and their mothers who might be in a similar situation.
“Undoubtedly other individuals will seek to align themselves with Begum’s position, but it is unlikely that this judgment would apply to everyone who left the UK to join Da’esh,” he said.
Begum, who married an Dutch-born Isil fighter and is currently living in a refugee camp in northern Syria, has previously said she had “no regrets” over her time with Isil and sparked outrage by claiming the Manchester terror attack was "justified.”
She is challenging the removal of her UK citizenship on the basis that the Government’s claim she has dual UK-Bangladesh nationality is wrong because Bangladesh is refusing to accept her and she has never lived there. Britain would be in breach of international law if found to have made her stateless.
If she returns, ministers fear she will never leave or be extradited as Bangladesh has not only rejected her citizenship but also has the death penalty for terrorists. “Regardless of the final outcome to her case, if Ms Begum was to come to this country, it will prove impossible to subsequently remove her,” said Mr Javid.
Richard Walton, Senior Fellow at Policy Exchange and former Head of Counter-Terrorism Command in the Metropolitan police, said: “The threat that Begum poses should not be underestimated. If she is allowed to return, she will present a very real challenge to Scotland Yard’s Counter Terrorism Command.
'She would likely be arrested and charged with terrorism offences but the evidence against her could be weak, owing to the difficulties of obtaining and seizing evidence in war zones.
'If released, she would present an on-going threat and would need to be subject to rigorous monitoring costing the state hundreds of thousands of pounds over months and years”
However, former defence minister Tobias Ellwood said Britain should take her back: “We should follow the American example and say it's a British problem. She was indoctrinated in Britain, the idea that we just palm her off is ignoring our responsibilities to look after British nationals.”
Five Britons stripped of citizenship who may benefit
Islamic convert Jack Letts, 25, dubbed Jihadi Jack, dual UK-Canadian citizenship, left UK to join Isil after dropping out of school
Jihadi brides Reema Iqbal, 30, and sister Zara, dual UK-Pakistani citizenship, left east London for Syria in 2013
Law student Ashraf Mahmud Islam, 23, dual UK-Bangladeshi citizenship, left London for Syria in 2015
Mahdi Hashi, 30, dual UK-Somali citizenship, was jailed by a New York court for supporting the terrorist organisation al Shabaab