Jihadi bride, Shamima Begum, has had her British citizenship revoked by the Home Office.
The 19-year-old, who had expressed the desire to return to the UK with her newborn son, will now be banned from entering the country.
It is understood the teenager, who grew up in Bethnal Green east London, has dual Bangladeshi nationality, meaning the move will not render her stateless.
In a letter sent to her family in east London, officials said the Home Secretary, Sajid Javid, had made the decision in "light of the circumstances".
The letter read: "Please find enclosed papers that relate to a decision taken by the Home Secretary, to deprive your daughter, Shamima Begum, of her British citizenship.
The letter went on to urge Begum's family to make the teenager aware of the decision, but added that she had a right to appeal.
In a statement the family's lawyer said they were very disappointed by the move.
Tasnime Akunjee said: "[The] Family are very disappointed with the Home Office's intention to have an order made depriving Shamima of her citizenship. We are considering all legal avenues to challenge this decision."
Shamima Begum will be effectively now be banned from entering the UK.
Javid has the power to exclude any person suspected of being involved in terrorist activity on the basis that that their presence in the UK is not conducive to the public good.
Begum travelled to Syria from her home in east London in December 2014 and married an Isis fighter.
Her first two children died, and last week she was discovered in a refugee camp where she issued a plea to return to Britain.
But despite saying she wants to bring her baby son up in the peace and security of the UK, she has insisted she has no regrets about travelling to Syria.
She has also been criticised for likening the deaths of 22 people in the Manchester Arena terror attack to the civilians being bombed in Isis territory.
Earlier the Met Commissioner, Cressida Dick, had said Begum could be arrested and potentially charged if she ever returned to the UK.
But she also acknowledged that travelling to Syria was not an offence in itself, and said the police would need evidence that she had been involved in crime or terrorism in order to bring charges.
It comes as a report suggests that many of the girls who have travelled to join Isis are far from vulnerable and naive.
A study by the Henry Jackson Society found evidence that while boys tended to join Isis under the influence of family members, girls were more likely to have sought out extremist material on their own.
Analysis of 20 cases found that Isis brides tended to be self-radicalised and motivated by the prospect of marrying a man of their choice.
Begum has insisted that while living in the self declared caliphate her only role was as a housewife and mother and there is no evidence of her having done anything wrong.
Speaking about the case, Dick said if the teenager returned to the UK she could face questioning but that the current law might not be sufficient to see her prosecuted.
The Commissioner explained: "If she does, under whatever circumstances, arrive at our borders somebody in her type of circumstances could expect to be spoken to and if there is the appropriate necessity, to be potentially arrested and certainly investigated."
"If that results in sufficient evidence for a prosecution then it will result in sufficient evidence for a prosecution. The officers will deal with whatever they are confronted with."
The Commissioner added: "If there is insufficient evidence for a prosecution it is our job to look at the threat they pose if they are returning from Syria and we do that with every single person who comes back from Syria and then manage the risk with colleagues in the (security and intelligence) agencies."
Dick went on: "Some people returned from that area in the early days who had almost certainly done nothing other than humanitarian aid work – we talked to them and assessed their risk. Many people have come back and just gone on with peaceful lives."
When Begum first travelled to Syria, Sir Mark Rowley, who was then the head of UK counter terrorism, suggested the girls might be treated as victims of grooming, but Dick said: "We're a long way down the road since then."