Ms Begum travelled to Syria in 2015 – at the age of 15 – before her citizenship was revoked on national security grounds shortly after she was found in a Syrian refugee camp in February 2019.
Earlier this year, the now-24-year-old lost a challenge against the decision at the Special Immigration Appeals Commission (SIAC).
Ms Begum’s lawyers brought a bid to overturn this decision at the Court of Appeal, with the Home Office opposing the challenge.
At the start of the hearing on Tuesday, Samantha Knights KC told the court the government had failed to consider the legal duties owed to Ms Begum as a potential victim of trafficking or as a result of “state failures” in her case.
She said in written submissions: “The appellant’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.”
But at the hearing in London on Wednesday, Sir James Eadie KC, for the department, said that decisions over whether someone is a victim of trafficking or whether they should be deprived of their citizenship “have fundamentally different bases and roles”.
He continued: “The focus in the trafficking regime is on the protection of the individual and there’s really no countervailing public interest at that point.
“But here the regime is different, the regime in operation is the deprivation regime and the rationale is entirely different, it is the protection of the public at large.”
The barrister later said the “key feature” of Ms Begum’s case was national security.
Sir James continued 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.”
During the appeal bid, Ms Begum’s lawyers said the UK has failed to have a “full and effective” investigation into how Ms Begum was allegedly trafficked.
In its ruling earlier this year, SIAC concluded there were “arguable breaches of duty” by state bodies – including the Metropolitan Police, Tower Hamlets council and Ms Begum’s school – in not preventing her from travelling to Syria.
Ms Knights told the Court of Appeal at the start of the three-day hearing that these “failures” could have also been unlawful and contributed to Ms Begum’s trafficking.
However, Sir James said SIAC was right to find there was “no direct connection between any potential failures, by other public authorities, in 2015” and Mr Javid’s decision to deprive Ms Begum of her citizenship.
The hearing in London is expected to conclude on Thursday, with the final day of the proceedings held in private.
At the end of Wednesday’s session, the Lady Chief Justice Lady Carr said: “We anticipate we will be reserving both open and closed decisions.”