Shamima Begum’s British citizenship should be restored because she cannot have a “fair and effective appeal” against the Government’s decision to strip her of it, the Court of Appeal has heard.

Ms Begum, now 20, was one of three east London schoolgirls who travelled to Syria to join so-called Islamic State (IS) in February 2015, and lived under IS rule for more than three years.

She was found, nine months pregnant, in a Syrian refugee camp in February last year, prompting then home secretary Sajid Javid to revoke her British citizenship later that month.

Last year, Ms Begum took legal action against the Home Office at the High Court and the Special Immigration Appeals Commission (SIAC), a specialist tribunal which hears challenges to decisions to remove someone’s British citizenship on national security grounds.

In February, SIAC ruled that the decision to revoke Ms Begum’s British citizenship did not render her stateless, and was therefore lawful, as she was “a citizen of Bangladesh by descent” at the time of the decision.

The tribunal also found that Ms Begum “cannot play any meaningful part in her appeal and that, to that extent, the appeal will not be fair and effective”, but ruled that “it does not follow that her appeal succeeds”.

Ms Begum’s challenge to the Home Office’s decision to refuse to allow her to enter the UK in order to effectively pursue her appeal was also rejected.

Opening Ms Begum’s case at a remote hearing on Thursday, Tom Hickman QC said the key issue in her appeal was whether the absence of “a fair or effective means of challenging the decision to deprive her of her British citizenship” made the decision unlawful.

He told the court: “It is a basic principle of our law that executive decisions cannot stand where the requirements of natural justice are not complied with.”

Mr Hickman said that “in the present case there is a manifest breach of natural justice”, and that Ms Begum’s appeal against the deprivation of her citizenship should be allowed because her appeal “cannot be pursued in a manner that satisfies even minimum requirements of fair procedure”.

He also said that Mr Javid had been informed that Ms Begum could not have a fair or effective appeal when he took the decision to revoke her British citizenship.

Mr Hickman said Ms Begum’s case was “the first case in which SIAC has held that an appellant cannot have a fair and effective appeal”.

He added that, nonetheless, SIAC “rejected the appellant’s claim for leave to enter the country” to pursue her challenge to the decision to revoke her British citizenship and had “suggested … that the appellant’s appeal might have to be stayed indefinitely or even struck out altogether”.

“In other words,” Mr Hickman said, “the consequence of the appellant not being able to have a fair and effective appeal means that the Secretary of State’s decision stands indefinitely and possibly forever without there ever having been a judicial decision on the merits (of Ms Begum’s appeal).

“That, we say, piles unfairness upon unfairness and is wrong in law.”

Mr Hickman pointed out that Ms Begum, who remains in the al-Roj camp in Syria, was only 15 when she left the UK, saying: “She had not even taken her GCSE exams.”

He added: “The only things that are clear are that Shamima Begum was a child when she left the UK and had been influenced to do so.”

Sir James Eadie QC, representing the Home Office, said in written submissions: “The fact that the appellant could not fully engage with the statutory appeal procedure was a result of her decision to leave the UK, travel to Syria against Foreign and Commonwealth Office advice and align with ISIL.

“This led to her being held in conditions akin to detention in a foreign state at the hands of a third party, the Syrian Defence Force.

“It was not the result of any action by the Secretary of State and the deprivation decision did not have any causative impact on the appellant in this respect.”

Sir James added that Ms Begum had been able to speak to her lawyers, and argued that “the fact that it might not be possible to mirror the level of access to legal advice that would be available if someone were at liberty in the UK does not mean the proceedings are unfair”.

Ms Begum was one of three schoolgirls from Bethnal Green Academy who left their homes and families to join IS, shortly after Sharmeena Begum – who is no relation – travelled to Syria in December 2014.

Kadiza Sultana and Amira Abase, then 16 and 15 respectively, and Ms Begum boarded a flight from Gatwick Airport to Istanbul, Turkey, on February 17 2015, before making 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 schoolfriends 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.

The two-day hearing before Lord Justice Flaux, Lady Justice King and Lord Justice Singh is being live-streamed on the judiciary’s YouTube channel, and it is expected that the Court of Appeal will reserve its judgment to a later date.