The family of Shamima Begum, who joined the Islamic State group in Syria aged 15, say they plan to challenge the move to strip her of UK citizenship.
Her family’s lawyer, Tasnime Akunjee, said they were considering “all legal avenues” to contest the decision.
Government sources said it was possible to strip the 19-year-old of British nationality as she was eligible for citizenship of another country.
Ms Begum, who left east London in 2015, had said she wanted to return home.
She was found in a Syrian refugee camp last week after reportedly leaving Baghuz – IS’s last stronghold – and gave birth to a son at the weekend.
A Home Office spokesman said the department did not comment on individual cases but decisions to remove citizenship were “based on all available evidence and not taken lightly”.
He said: “In recent days the home secretary has clearly stated that his priority is the safety and security of Britain and the people who live here.”
Bangladesh link
Ms Begum is believed to be of Bangladeshi heritage but when asked by the BBC, she said she did not have a Bangladesh passport and had never been to the country.
Lord Carlile, a former independent reviewer of terrorism legislation, said that if Ms Begum’s mother was a Bangladeshi national – as is believed to be the case – under Bangladesh law Ms Begum would be too.
However, the family’s lawyer Mr Akunjee told the Independent that the Bangladeshi government “does not know who she is”.
He said: “Our position is that to all practical purposes she has been made stateless.”
Under the 1981 British Nationality Act, a person can be deprived of their citizenship if the home secretary is satisfied it would be “conducive to the public good” and they would not become stateless as a result.
In an interview with the BBC on Monday, Ms Begum said she never sought to be an IS “poster girl” and now simply wished to raise her child quietly in the UK.
On the question of Ms Begum’s son, a child born to a British parent before they are deprived of their citizenship would still be considered British.
While it would theoretically be possible for the UK to then remove citizenship from the child, officials would need to balance their rights against any potential threat they posed.
ITV News obtained the letter sent to Ms Begum’s mother, asking her to inform her daughter of the government’s decision.
Dal Babu, a former Metropolitan Police chief superintendent and friend of Ms Begum’s family, said they were “very surprised” by what seemed to be a “kneejerk reaction” by the Home Office.
Stressing that Ms Begum had never been to Bangladesh, Mr Babu said: “It seems to be a bizarre decision and I’m not entirely sure how that will stand up legally.”
Conservative MP George Freeman said the move was a “mistake” that would set a “dangerous precedent”.
But Tory MP Robert Halfon backed the Home Office move, saying it was “absolutely the right decision”.
