Shamima Begum, who left the UK for Syria as a teenager to join the Islamic State group, was a victim of human trafficking for the purposes of sexual exploitation, a court has heard.
Ms Begum travelled to Syria in 2015, with her citizenship stripped on national security grounds in 2019.
A five-day immigration hearing is considering a new attempt to challenge the removal of her UK citizenship.
The Home Office insists she continues to pose a threat to national security.
The case is being heard at the Special Immigration Appeals Commission (SIAC), which can hear national security evidence in secret if necessary.
Lawyers for Ms Begum, now 23, told the court that a decision by the then home secretary, Sajid Javid, to remove her British citizenship was unlawful, as it did not consider whether she had been a child victim of trafficking.
She remains in a camp controlled by armed guards in northern Syria, nearly eight years after running away from home in London aged 15 alongside two other east London schoolgirls – Kadiza Sultana, 16, and 15-year-old Amira Abase.
In August, a BBC News investigation revealed Ms Begum was smuggled into Syria by an intelligence agent who was working for Canada at the time. The Canadian government has said that it will “follow up” on the allegations.
Once in Syria, she married a Dutch recruit and lived under IS rule for more than three years. In 2019, she was found by The Times newspaper, nine months pregnant, in a Syrian refugee camp. Her baby later died of pneumonia and Ms Begum said she had previously lost two other children.
In an interview with BBC News in September 2021, Ms Begum said she would regret joining the Islamic State group (referred to as IS, ISIS and ISIL) for the rest of her life and offered to help the UK fight terrorism.
Ms Begum previously said the 2017 Manchester Arena bombing, in which 22 people – some of them children – were killed in a bombing claimed by IS, was similar to military strikes on IS strongholds and called the terror attack “retaliation”.
Ms Begum’s lawyers argued in court on Monday that the evidence she was a child victim of trafficking is “overwhelming” and that “depriving her of her British citizenship was a disproportionate means of addressing any alleged national security risk”.
Samantha Knights KC said: “At its heart this case concerns a British child aged 15 who was persuaded, influenced and affected with her friends by a determined and effective ISIS propaganda machine.”
She added that “inadequate and over hasty steps were taken to deprive her of her citizenship forever within a week of her being interviewed by a UK journalist”.
Lawyers for the Home Office argued the decision to deprive Ms Begum of her citizenship was correct, saying that when she fled from IS territory “she left only for safety and not because of a genuine disengagement from the group”.
They told the court that, in “multiple press interviews” before Mr Javid made his decision, she expressed “no remorse and said she did not regret” joining a terrorist organisation – also arguing she had acknowledged she was “aware of the nature of the group when she travelled”.
A formal assessment made by the UK security service MI5 says that people who travelled to IS controlled “will have been radicalised and exposed to ISIL’s extremism and violence”, with the security service further assessing anyone returning “will present a national security threat to the UK”.
MI5 assessed that her “activities prior to and during her travel to Syria demonstrated determination and commitment to aligning with ISIL”.
The Home Office also said that information given to the police in November 2015 “indicated that Ms Begum did not share Ms Sultana’s desire to return home and remained supportive of ISIL following her arrival in ISIL territory”.
The hearing, which is expected to last five days, continues.