The government has deprived Shamima Begum of her British citizenship, in a move the teenager’s lawyer says makes her and her baby effectively stateless.
The security minister previously said the Isis supporter had a right to return to the UK from Syria as a British national, and the justice secretary said she could not be made stateless.
The document, addressed to Begum’s mother, said the decision was taken “in light of the circumstances of your daughter”.
“I would be very grateful if you could ensure the home secretary’s decision is brought to her attention, along with her right to appeal,” it added.
The family’s lawyer, Tasnime Akunjee, told The Independent Begum had been a British national and had “never had a Bangladeshi passport”.
He said that although Bangladeshi law affords citizenship to foreign residents with parents from the country, the Dhaka government “does not know who she is”.
“It’s an entitlement,” Mr Akunjee said: “Our position is that to all practical purposes she has been made stateless.”
The lawyer also accused the government of making Begum’s days-old son stateless, and leaving them both “stuck” in a Syrian detention camp.
He said the family would launch a legal challenge, adding: “This is a delaying tactic. The government is not going to win this, there is case law saying people in these circumstances are stateless and we will win but how long will that take?”
The Home Office declined to comment on Mr Akunjee’s allegations but said: “We do not make people stateless.”
Ms Begum’s family is of Bangladeshi descent and in Syria she married a Dutch foreign fighter.
The citizenship of their child is unclear, and in a previous case a baby born to a female Isis supporter in Syria was only given British citizenship after a DNA test proving her parentage.
Government guidance states that the home secretary can deprive citizenship for the “public good” if a person can apply for alternative nationality.
“This action may only be taken if the Secretary of State has reasonable grounds for believing that the person is able, under the law of a country outside the United Kingdom, to become a national of that country,” says a document published last year.
“In practice, this power means the Secretary of State may deprive and leave a person stateless if that person is able to acquire (or reacquire) the citizenship of another country.”
Mr Javid previously suggested he would seek to prevent Begum’s return to the UK, but the security minister had said she was legally entitled to.
“As a British citizen she has a right to come home here,” Ben Wallace said last week. “We are obliged to make sure our citizens have rights, no matter who they are.”
Begum left the UK aged 15 in 2015 with two other friends from Bethnal Green Academy, joining a fourth girl from the same school in Syria.
In media interviews she has said she has “no regrets” about joining Isis and called the Manchester attack “justified”.
On Saturday, the justice secretary David Gauke said the government “can’t make people stateless”.
In November the government was found to have acted unlawfully in removing the citizenship of two British alleged Islamists.
The government said the men were eligible for Bangladeshi citizenship, but the Special Immigration and Appeals Commission found they had been made stateless.
In 2017, the government was found to have unlawfully stripped a third person of Bangladeshi descent of British citizenship.
The London-born woman, known as G3, was alleged to be an Isis member and was detained by Turkish authorities on the Syrian border with her two young children in 2016.
Use of citizenship deprivations has been increasing. In 2017, the government stripped 104 people of their British citizenship, compared to just 14 people in 2016.
The measure has been used for high-profile terrorists, including alleged members of the British Isis cell known as the Beatles, as well as grooming gang members.