The decision to remove Shamima Begum's British citizenship was "unlawful", a court has heard, as her latest appeal against the decision begins.
Ms Begum travelled to Syria in 2015 to join Islamic State, when she was aged 15, and her UK citizenship was revoked on national security grounds shortly after she was found in a refugee camp in February 2019.
Earlier this year, Ms Begum, now 24, lost a challenge against the decision at the Special Immigration Appeals Commission (SIAC), meaning she would not be able to return to the UK.
Delivering the ruling in February, Mr Justice Jay said that while there was "credible suspicion that Ms Begum was recruited, transferred and then harboured for the purpose of sexual exploitation", this did not prevent then-home secretary Sajid Javid from removing her citizenship.
At the Court of Appeal in London on Tuesday, her lawyers began another bid to overturn the decision - which the Home Office is opposing.
Her legal team claims the Home Office failed to consider the legal duties owed to Ms Begum as a potential trafficking victim.
Samantha Knights KC said in written submissions: "[Ms Begum's] trafficking was a mandatory, relevant consideration in determining whether it was conducive to the public good and proportionate to deprive her of citizenship, but it was not considered by the Home Office.
"As a consequence, the deprivation decision was unlawful."
Addressing the SIAC's conclusion that there were "arguable breaches of duty" by state bodies including the Metropolitan Police, Tower Hamlets council and Ms Begum's school, Ms Knights said these "failures" could have been unlawful and contributed to Ms Begum being trafficked.
Lawyers for the Home Office have told the court that the SIAC outcome was correct.
Sir James Eadie KC said in written submissions: "The fact that someone is radicalised, and may have been manipulated, is not inconsistent with the assessment that they pose a national security risk.
"Ms Begum contends that national security should not be a 'trump' card. But the public should not be exposed to risks to national security because events and circumstances have conspired to give rise to that risk."
The hearing is expected to last three days with the decision to follow at a later date.