A Bangladeshi-origin British woman, who fled the UK as a 15-year-old schoolgirl to join the Islamic State (ISIS) terrorist network, on Wednesday lost a legal bid before a specialist tribunal to regain her British citizenship and return to the country.

Now aged 23, London-born Shamima Begum had challenged then Home Secretary Sajid Javid's decision to revoke her British citizenship on national security grounds in 2019 at the Special Immigration Appeals Commission (SIAC).

Justice Robert Jay ruled that concerns raised by her lawyers over her "sexual exploitation” and breaches of duty by certain state bodies did not trump the home secretary's legal duty to make a national security decision to strip Begum of her British nationality.

"There is some merit in the argument that those advising the secretary of state see this as a black and white issue, when many would say that there are shades of grey," said Justice Jay.

"In my mind and that of colleagues, it is not conceivable that even a 15-year-old... an intelligent, articulate and presumably critically thinking individual – would not know what (ISIS) was about. In some respects, I do believe she knew what she was doing and had agency in doing so," the judgment notes.

The British government has claimed Begum could seek a Bangladeshi passport given her heritage but her family has argued that she is British and has never held Bangladeshi citizenship.

The specialist tribunal accepted that Javid’s conclusion to revoke her British citizenship was "an integral part of the overall national security assessment carried out by the Security Service" and therefore not a matter for the court.

"If asked to evaluate all the circumstances of Begum's case, reasonable people with knowledge of all the relevant evidence will differ, in particular in relation to the issue of the extent to which her travel to Syria was voluntary and the weight to be given to that factor in the context of all others," notes Justice Jay.

"Likewise, reasonable people will differ as to the threat she posed in February 2019 to the national security of the United Kingdom, and as to how that threat should be balanced against all countervailing considerations. However, under our constitutional settlement these sensitive issues are for the secretary of state to evaluate and not for the Commission,” he said.

Begum, referred to as an ISIS bride for marrying a Dutch member of the terror network in Syria, gave birth to three children – all of whom later died. She and two fellow teenage schoolgirls at Bethnal Green Academy travelled from east London to Syria in 2015.

At a five-day SIAC hearing in November last year, Begum's lawyers said that she was "recruited, transported, transferred, harboured and received in Syria for the purposes of 'sexual exploitation' and 'marriage' to an adult male".

Justice Jay found that there was “credible suspicion” Begum was a victim of trafficking to Syria.

"The motive of bringing her to Syria was sexual exploitation for which, as a child, she could not give valid consent,” he said, during a brief hearing on Wednesday.

However, the commission ultimately concluded the home secretary was not formally required to consider whether Begum was a victim of trafficking when he removed her citizenship.

The UK Home Office welcomed the ruling and said in a statement: “We are pleased that the court has found in favour of the government’s position in this case.

"The government’s priority remains maintaining the safety and security of the UK and we will robustly defend any decision made in doing so.” Last year, the UK Supreme Court upheld the decision to bar the now 23-year-old from returning to the UK. Begum currently lives in a detention camp in northern Syria and has been fighting to return to her home in east London, claiming the only law she broke was to travel to Syria and had not participated in any ISIS atrocities.

Her legal team has indicated plans to mount further challenges to restore her British citizenship.

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