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In the letter, which was obtained by ITV News, Shamima's mother was told: "Please find enclosed papers that relate to a decision taken by the Home Secretary, to deprive your daughter, Shamima Begum, of her British citizenship.
"In light of the circumstances of your daughter, the notice of the Home Secretary's decision has been served of file today (19th February), and the order removing her British citizenship has subsequently been made."
A statement from the family, shared by their lawyer, Mohammed T Akunjee, said they are hoping to challenge the decision.
Shamima was found in a Syrian refugee camp by the Times newspaper last week, having reportedly left Baghuz, IS's last stronghold in the country.
Last weekend she then gave birth to a baby boy, who she hoped to bring back to the UK with her.
A statement from her family said: "We the family of Shamima Begum have been informed that Shamima has given birth to her child, we understand both she and the baby are in good health.
"As yet we have not had direct contact with Shamima, we are hoping to establish communications with her soon so that we can verify the above."
Following an interview with the BBC, Shamima hit headlines for her comments surrounding the the 2017 Manchester Arena attack, which killed 22.
Comparing what happened to military assaults on Syria, she said: "It's one thing to kill a soldier, it's fine, it's self-defence. But to kill people like women and children just like the women and children in Baghuz who are being killed right now unjustly by the bombings - it's a two-way thing really because women and children are being killed back in the Islamic State right now.
"It's kind of retaliation. Their justification was that it was retaliation so I thought, okay, that is a fair justification."
She did, however, add she felt it was 'wrong' that innocent people - including children - got killed.
"I just want forgiveness really, from the UK," she said.
"Everything I've been through, I didn't expect I would go through that."
Actor Danny Dyer weighed in on the matter during a recent interview on Good Morning Britain, where he said she should be allowed to return to the UK.
"I feel she needs a chance, maybe to explain what was going on and maybe we can understand a bit more how they got to her and how she thought it was the right move," he said.
"She's still a young girl, who was looking after her? Maybe we can learn from it, I don't know."
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