Senior judges ruled on Friday that Shamima Begum should not be allowed to return to the UK to fight for her citizenship.
But who is Shamima Begum and why is the Home Office appealing the court's decision to allow her to return?
Ms Begum was one of three east London schoolgirls who left their homes and families to join the so-called Islamic State (IS).
The group left shortly after Sharmeena Begum – who is no relation – travelled to Syria in December 2014.
The three Bethnal Green Academy pupils - Ms Begum, Kadiza Sultana and Amira Abase, then 16 and 15 respectively - boarded a flight from Gatwick Airport to Istanbul on February 17, 2015.
From there, the trio made their way to Raqqa in Syria.
Ms Begum claims she married Dutch convert Yago Riedijk 10 days after arriving in IS territory - with all three of her school friends also reportedly marrying foreign IS fighters.
She told The Times last February that she left Raqqa in January 2017 with her husband but her children, a one-year-old girl and a three-month-old boy, had both since died.
Her third child died shortly after he was born in 2019.
In February 2019, ITV News Security Editor Rohit Kachroo showed Ms Begum a copy of the letter sent to her family in the UK from the Home Office.
In it, she is informed that she is being stripped of her British citizenship.
Then then-teenager said: "I don't know what to say. I'm not that shocked but I am a bit shocked."
It was then believed that the fact Ms Begum's father was from Bangladesh meant she could claim Bangladeshi citizenship.
But in May 2019, Bangladesh's foreign minister told ITV News Ms Begum is "not our problem".
Abdul Momen warned if Ms Begum did go to Bangladesh - after being stripped of her British citizenship - she could be hanged for terrorism.
Ms Begum then took legal action against the UK Home Office - claiming the decision to strip her of her citizenship was unlawful because it rendered her stateless and exposed her to a real risk of death or inhuman and degrading treatment.
In February of this year, the Special Immigration Appeals Commission (SIAC) ruled the decision was lawful as Ms Begum was “a citizen of Bangladesh by descent” at the time of the decision.
Ms Begum’s challenge to the Home Office’s decision to refuse to allow her to enter the UK to effectively pursue her appeal was also rejected in February.
The Court of Appeal said last year that "the only way in which she can have a fair and effective appeal is to be permitted to come into the United Kingdom to pursue her appeal".
Judges ruled that Ms Begum should be allowed to return to the UK to challenge the deprivation of her British citizenship.
But on 26 February 2021 the Supreme Court decided Ms Begum should not be allowed to return to the country to pursue an appeal against the removal of her British citizenship.
Announcing the decision, Lord Reed said: “The Supreme Court unanimously allows all of the Home Secretary’s appeals and dismisses Ms Begum’s cross-appeal.”