Shamima Begum, who joined Islamic State in Syria aged 15, is to lose her UK citizenship, the family's lawyer says.
In this file photo taken on February 22, 2015 Renu Begum, eldest sister of missing British girl Shamima Begum, holds a picture of her sister while being interviewed by the media in central London Photo: AFP
The 19-year-old ran away to Syria as a schoolgirl in 2015 to join the Islamic State and has since given birth.
She had said she wanted to return to the UK to have the baby was told she could face prosecution if she returned.
In a Twitter statement, lawyer Tasnime Akunjee said the family were "disappointed" with the decision and were considering "all legal avenues" to challenge it.
ITV News said Ms Begum's mother had received a letter from the Home Office on Tuesday.
The letter said Home Secretary Sajid Javid had ordered the move and it had already been processed.
It asked the mother to let her daughter know of the decision and to inform her she has a right to appeal.
Mr Akunjee has previously told the BBC that the baby was "no threat" and that legally Ms Begum was allowed to return as a British citizen.
Earlier the Home Secretary Sajid Javid had said he would do everything in his power to prevent her coming back.
He said more than 100 dual nationals have already lost their UK citizenship after travelling in support of terrorist groups.
Under international law, the UK has an obligation to allow a British citizen who does not have a claim to another nationality to return.
In an interview with the Times newspaper, Ms Begum, who married an IS fighter, showed little remorse for her involvement with the terror group, saying she was not fazed by seeing "beheaded heads" in bins.
"I don't regret coming here," she told Times journalist Anthony Loyd, who found her in the camp.
"I'm not the same silly little 15-year-old schoolgirl who ran away from Bethnal Green four years ago."
"The caliphate is over. There was so much oppression and corruption that I don't think they deserved victory," she said.
"I just want to come home to have my child. I'll do anything required just to be able to come home and live quietly with my child."
- BBC