Shamima Begum, who joined the Islamic State group in Syria aged 15, is to lose her UK citizenship.
Whitehall sources said it was possible to strip the 19-year-old of British nationality as she was eligible for citizenship of another country.
Her family’s lawyer, Tasnime Akunjee, said they were “disappointed” with the decision and were considering “all legal avenues” to challenge it.
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.
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.
ITV News obtained the letter sent to Ms Begum’s mother, asking her to inform her daughter of the decision.
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.
Ms. Begum said she travelled to Syria with her sister’s UK passport but it was taken from her when she crossed the border.
She is believed to be of Bangladeshi heritage but when asked by the BBC, she said did not have a Bangladesh passport and had never been to the country.
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.
Ms. Begum gave birth to a baby boy last weekend, having previously lost two children.
Her husband, a Dutch convert to Islam, is thought to have surrendered to a group of Syrian fighters about two weeks ago.
Ms Begum has the right to appeal the Home Office’s decision.