Lawyer Mohammed Akunjee said in a statement on Twitter that her family were "very disappointed with the Home Office's intention to have an order made depriving Shamima of her citizenship."
He added that they were "considering all legal avenues to challenge this decision."
ITV News reported that Begum's mother had received a letter from the Home Office which said an order to remove her citizenship had been made. The letter asked Begum's family to make her aware of the decision and of her right to appeal.
In a statement, the Home Office said it would not comment on individual cases. But a spokesperson added that the Home Secretary can deprive someone of their British citizenship "where it would not render the individual stateless."
It's unclear whether Begum has citizenship of more than one country. CNN has contacted the family's lawyer for further comment.
Begum was 15 when she left flew out of London's Gatwick Airport with two classmates and traveled to Syria. The young women, all from the Bethnal Green Academy in east London, were to join another classmate who had traveled to Syria months earlier.
The Times reported she was nine months pregnant and that she wanted to come home to have her child. She said she had two other children during her time in Syria who died in infancy from malnutrition and illness.
Now in a Syrian refugee camp of 39,000 people in al-Hawl, Begum told The Times she had no regrets about traveling to Syria but told the paper that "the caliphate is over."
Days later, she spoke to Sky News from the camp and hours after giving birth. During the interview she said she was aware of ISIS beheadings and other brutality before she left to join the group, and was "OK" with it.
"Yeah I knew about those things and I was OK with it. I started becoming religious just before I left. From what I heard, Islamically that is all allowed, so I was OK with it," Begum told Sky News.
However Begum, who married an ISIS fighter, said she was only a housewife during her time in Syria.
"I never did anything dangerous, I never made propaganda, I never encouraged people to come to Syria," she said.
UK Home Secretary Sajid Javid said last Friday that he would "not hesitate" to prevent the return of those who joined terror organizations like ISIS.
"My message is clear: if you have supported terrorist organizations abroad I will not hesitate to prevent your return," he said.
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