Shamima Begum, who fled the UK to join the Islamic State terror group in Syria aged 15, has been stripped of her British citizenship.
Home Secretary Sajid Javid ordered the move against the 19-year-old Londoner who wants to return to the UK with her newly-born child as the so-called caliphate crumbles.
She was part of a trio of girls from Bethnal Green Academy to travel to the war-torn nation to support the terror group in February 2015.
Her family's lawyer, Tasnime Akunjee, described them as "very disappointed" over the move and said they were "considering all legal avenues to challenge this decision".
The Home Office said such decisions are "not taken lightly" and are carried out "in order to protect this country".
ITV News reported a letter from the department 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," it read.
"In light of the circumstances of your daughter, the notice of the Home Secretary's decision has been served of file today (19 February), and the order removing her British citizenship has subsequently been made."
The letter asked the mother to inform her daughter of the decision, as well as her right to appeal.
The move comes after the teenager returned to the public eye when she was found heavily pregnant living in a refugee camp in northern Syria.
She gave birth to a boy over the weekend, having already lost two children, and made pleas for forgiveness and to be accepted back in the UK.
Metropolitan Police Commissioner Cressida Dick has signalled she could be arrested and investigated if she returns to Britain.
When Ms Begum left the UK, the then chief of counter-terror policing Sir Mark Rowley suggested that she might be treated as a victim of grooming.
But on Tuesday Ms Dick said: "We're a long way down the road since then."
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.
Government guidance from 2017 states that the Home Secretary has the power to order the deprivation if it would be "conducive to the public good", as long as they are not left without any citizenship.
A Home Office spokesman said: "In order to protect this country, he 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."
Earlier, Mr Javid told the House of Commons he had the power of "stripping dangerous dual nationals" of their British citizenship, adding: "Over 100 people have already been deprived in this way."
Conservative former minister George Freeman criticised the move as a "mistake" that will set a "dangerous precedent".
The MP for Mid Norfolk tweeted: "She was born here, educated here & is our responsibility. We should defend our system & she should be brought back to face the UK courts."
The Liberal Democrats called for Ms Begum to be allowed to return to the UK to face a prosecution for membership of a terrorist group.
"It is not only hard to see Ms Begum and her baby as constituting a serious threat to national security, but it also seems a huge wasted opportunity," home affairs spokesman Ed Davey said.
"We can learn lessons as to why a young girl went to Syria in the first place, lessons which could improve Britain's security by helping us prevent this happening again."
Human rights campaign group Liberty said revoking a person's citizenship was a power that "must not be wielded lightly".
A spokesman said: "The government has an array of powers available to it to deal with people suspected of involvement in terrorism - including the criminal law. Taking away a person's citizenship is one of the most serious among them and must not be wielded lightly.
"The Home Office must act proportionately and in accordance with its obligations under international law. Crucially, its response must include a serious look at how a 15 year-old girl was apparently able to leave the country to join a proscribed group without effective action being taken to safeguard her."
Robert Halfon, Conservative MP for Harlow and chairman of the Commons Education Committee, backed the Home Office move, tweeting: "Absolutely the right decision from @ukhomeoffice @sajidjavid".
On Monday, in an interview with the BBC, Ms Begum compared the Manchester Arena bombing to military strikes on Isis strongholds, calling the terror attack "retaliation".
There are currently plans to change the law to make travelling to certain terror hotspots a criminal offence, but this would not apply retrospectively to Ms Begum.
Around 425 suspected jihadi fighters are thought to have returned to the UK from Syria so far.
By Sam Blewett and Lizzie Roberts, Press Association