Mohammed Tasnime Akunjee said in a Tweet on Tuesday that the family was “very disappointed with the Home Office’s intention to have an order made depriving Shamima of her citizenship”.
He said the family is “considering all legal avenues to challenge this decision”.
Begum fled from her East London home in 2015 alongside two female friends to join the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.
Now 19 years old, she says she has given birth to a baby and wants to come home.
Last week, in her first interview since leaving the UK, Begum told The Times newspaper that she was “not the same silly little 15-year-old schoolgirl who ran away from Bethnal Green four years ago”.
She refused to express regret over joining the group, however and added she had been “OK with” beheadings carried out by ISIL adherents because she had heard it was allowed under the Islamic law.
News about Begum and her desire to go back to Britain have ignited a debate in the UK about how to deal with citizens who joined ISIL and want to leave Syria now that the group is on the verge of collapse.
On Friday, British Home Secretary Sajid Javid said the British government might try to block her return.
“We must remember that those who left Britain to join Daesh [Islamic State] were full of hate for our country,” Javid said.
However, Justice Minister David Gauke said on Saturday that there could be legal problems if Begum’s wish to return was rejected. He told UK broadcaster Sky News that people should not be made stateless.
Officials have refused to rule out prosecuting Begum should she return.