London mayor Sadiq Khan said Shamima Begum should return to the UK to face justice in the criminal courts.
On Thursday, the Court of Appeal ruled that Ms Begum – one of three east London schoolgirls who travelled to Syria to join the so-called Islamic State group (IS) – should be allowed to return to the UK to fight the decision to remove her British citizenship.
Downing Street said the Government was “bitterly disappointed” by the ruling and the Home Office said it plans to appeal.
Mr Khan told the PA news agency on Friday: “I think people who commit criminal offences must face the music.
“While we’ve got courts in this country and judges who are some of the fairest in the world, I think if a British citizen commits an offence here or overseas they should face justice in the criminal courts.
“And if she (Begum) has committed a criminal offence I’m sure the jury will find her guilty, and justice will be served.
“What I’m not in favour of is us sub-contracting justice to another country.”
Ms Begum, now 20, travelled to Syria in February 2015 and lived under IS rule for more than three years before she was found, nine months pregnant, in a Syrian refugee camp in February last year.
Then-home secretary Sajid Javid revoked her British citizenship on national security grounds later that month.
Ms Begum took legal action against the Home Office, claiming the decision was unlawful because it rendered her stateless and exposed her to a real risk of death or inhuman and degrading treatment.
In February, the Special Immigration Appeals Commission (SIAC) – a specialist tribunal which hears challenges to decisions to remove someone’s British citizenship on national security grounds – 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.
On Thursday, the Court of Appeal ruled 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”.
Lord Justice Flaux – sitting with Lady Justice King and Lord Justice Singh – said: “Fairness and justice must, on the facts of this case, outweigh the national security concerns, so that the leave to enter appeals should be allowed.”
The judge found that “the national security concerns about her could be addressed and managed if she returns to the United Kingdom”.
A Home Office spokeswoman said: “This is a very disappointing decision by the court.
“We will now apply for permission to appeal this judgment, and to stay its effects pending any onward appeal.”
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 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.