Lawyers for Shamima Begum have gone before the UK Supreme Court to argue the government was acting illegally when it stripped her of her UK citizenship after she joined Daesh.
The Supreme Court will hear opposing arguments on whether she poses as a security threat and whether that outweighs her human rights.
Sir James Eadie, a lawyer representing the UK government, told the Supreme Court judges on Monday 23 November that Begum was "considered to pose a real and current threat to national security.''
In July the Court of Appeal in London ruled that Begum, who is now 20, should be present in Britain to hear the legal challenge. She is currently in a refugee camp.
But the government appealed and the Court of Appeal agreed the case should be referred up to the Supreme Court because it raised "points of law of general public importance."
Sir James, quoting an MI5 assessment, stated on Monday: "The fact is the threat is real and serious, despite the age of the individual when they travelled. The assessment is that those who have travelled to align and have aligned pose a clear and serious threat specifically on return."
He added: "The exposure of the public to an increased risk of terrorism is not justifiable or appropriate in this case on fairness grounds."
Sir James pointed out: "She married an IS fighter, lived in Raqqa - the capital of the self-declared caliphate - and remained with them for about four years until 2019 when she left from in effect the last pocket of IS territory in Baghuz."
In February 2019 the then Home Secretary Sajid Javid, and Prime Minister Theresa May revoked her citizenship on national security grounds.
Begum claimed she married a Dutch jihadist soon after arriving in Daesh territory. She lost two of his children and another new born baby died in the refugee camp near the Iraqi border last year.
In August a United Nations official warned that women who join Daesh are being “underestimated” by international security services due to gender stereotypes.
Begum, interviewed in a refugee camp by ITV News said at the time: “I am a bit shocked. It’s a bit upsetting and frustrating. I feel like it’s a bit unjust on me and my son.”
She started legal action to reclaim her British citizenship, arguing it was unlawful to make stateless and expose her to the risk of “death or inhuman and degrading treatment.”
During Monday's hearing Sir James referred to the case of K2, a British-Sudanese national who was stripped of his UK citizenship and permanently excluded from the UK. In 2017 the European Court of Human Rights ruled the UK government had not violated K2's rights under Article 8 of the Convention.
Begum’s parents were born in Bangladesh but she was born in London and Dhaka has refused to consider granting her Bangladeshi citizenship.
Lord Pannick QC, representing Begum, told the hearing the Special Immigration Appeals Commission (SIAC) had said that Shamima Begum could not play any part in her appeal against the Home Secretary's ruling if she was not present.
Lord Pannick said: "Her appeal is not subject to any relevant national security exception. We say that national security considerations cannot be an answer to the right to a meaningful right of appeal to SIAC."
He added: "We say there is no question of balance here. The Secretary of State cannot deprive her of her citizenship unless a means can be conferred on her to her right to appeal. Leave to entry is the only means available."
Lord Pannick said the current Home Secretary, Priti Patel, was entitled to bring legislation before Parliament to address the issues of national security and citizenship but he insisted that as the law stands now the government was acting illegally.
He also said: "Sir James claims that this is all Ms Begum's fault, she decided to go to Syria and stayed there of her own free will and she is the author of her own misfortunes. With respect that is no answer to the statutory right to a meaningful appeal which she enjoys."
Lord Pannick said he had no idea what evidence Ms Begum might bring forward but he speculated that she might claim that once she turned 18 she found it impossible to disobey the leaders of Daesh and leave the Caliphate.
*Daesh [ISI, ISIL, the Islamic State] - a terrorist organisation banned in Russia and many other countries.