Senior judges ruled Ms Begum should be allowed to return to the UK to fight the decision to remove her British citizenship.
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.
However, Downing Street said the Government was “bitterly disappointed” by the ruling and the Home Office said it plans to appeal.
Mr Khan said 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.”
From a look at Daily Mail comments her imminent return to UK soil, to face justice, is not welcome…at all