Video report by ITV News Security Editor Rohit Kachroo
Islamic State schoolgirl Shamima Begum has had her British citizenship revoked, her family has been told in a letter from the Home Office.
The letter, obtained by ITV News, was received by Ms Begum's mother on Tuesday.
"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 read.
"In light of the circumstances of your daughter, the notice of the Home Secretary's decision has been served of file today (19th February), and the order removing her British citizenship has subsequently been made."
The letter went on to urge Ms Begum's family to make her aware of the decision, and added that she had a right to appeal.
In a statement posted on Twitter, the family's lawyer, Tasnime Akunjee, said: "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."
Last week, the 19-year-old, then heavily pregnant, was found in a refugee camp in Syria where she said she wanted to come back to the UK.
On Sunday, she gave birth to a baby boy.
She and two school friends from Bethnal Green fled the UK via Turkey in 2015, where they made their way to Syria and were married off to Islamic State fighters.
Speaking to ITV News on Monday, the teenager insisted she was "not a threat" to the UK, despite having insisted she did not regret joining IS.
"I'm a 19-year-old girl with a newborn baby," she said..
"I don't have any weapons; I don't want to hurt anyone even if I did have weapons.
"He [Home Secretary Sajid Javid] has no proof that I'm a threat other than that I was in ISIS but that's it.
"I don't know how I would be seen as a danger.
"I'm not going to go back and provoke people to go to ISIS or anything, if anything I'm going to encourage them not to go because it's not all as it seems in their videos."
Is revoking Shamima Begum's citizenship legal?
Metropolitan Police Commissioner Cressida Dick Ms Begum signalled she could be arrested and investigated if she returns to Britain.
When she left the UK, the then chief of counter-terror policing Sir Mark Rowley suggested that she might be treated as a victim of grooming.
International law forbids nations from making people stateless by revoking their only citizenship, but it is possible Ms Begum, who is of Bangladeshi heritage, held dual citizenship.
Ms Begum however, does not hold a Bangladeshi passport and has never lived there.
A Home Office spokesperson said: "In order to protect this country, he [Mr Javid] has the power to deprive someone of their British citizenship where it would not render them stateless.
"We do not comment on individual cases, but any decisions to deprive individuals of their citizenship are based on all available evidence and not taken lightly."
Legal commentator Joshua Rozenberg told ITV News the Home Secretary can revoke British citizenship if it is seen to be for the "public good."
He said: "Under the British Nationality act the Home Secretary is entitled to deprive a person of citizenship if the Home Secretary thinks that deprivation is conducive to the public good, that it's a good thing to do.
"Now the Home Secretary can't do that if that would render the person stateless, but he says Shamima Begum has Bangladeshi nationality and therefore he's entitled to take away her British nationality."
What can Shamima Begum do now?
According to Mr Rozenberg, Ms Begum can appeal to a court called the Special Immigration Appeals Commission.
"There have been cases that have gone through and people have been able to show that they would be entitled to foreign nationality, indeed Bangladeshi nationality," he said.
"In that case they can't be deprived of their British citizenship so we can expect a battle to come."
What happens to her son?
While admitting there are a lot of grey areas around Ms Begum's case, Mr Rozenberg said her son is "presumably entitled to British citizenship".
He added since her son "certainly hasn't been registered as a Bangladeshi", he would "in principle would be allowed to come to this country".
"It seems clear that Shamima Begum has Bangladeshi nationality and therefore, she's not going to be rendered stateless if her British citizenship is taken away," he said.
"Her baby is in a different position. He's not entitled to Bangladeshi citizenship, she could register him but obviously she hasn't done that.
"So the baby is presumably entitled to come here to the United Kingdom. And if the baby can come, perhaps the mother can come too."
Ms Begum first spoke out about wanting to come back to Britain from the al-Hawl refugee camp in Syria.
Following her comments the Begum family had pleaded with the Government to treat her case as a "matter of urgency".
Prior to her citizenship being revoked Justice Secretary David Gauke reiterated the UK could not make people "stateless".
MI6 chief Alex Younger had also alluded to a similar outcome at a security conference in Munich.
While he refused to comment on specific circumstances he said: "British nationals have a right to come to the UK."