The mother of Shamima Begum has written to the Home Office to urge the department to reconsider its decision to strip the teenager of her British citizenship as an “act of mercy”.

Begum, from Bethnal Green, east London, was 15 when she and two other schoolgirls went to join Islamic State in February 2015. She has lost three children since travelling to Syria, including most recently her three-week-old son, Jarrah, on Friday.

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The home secretary, Sajid Javid, has faced severe criticism for his decision to strip the 19-year-old of British citizenship – a move that left her stateless and her baby in legal limbo.

A letter to the Home Office published on Twitter by the family’s lawyer said: “We write specifically on behalf of Mrs Begum, Shamima Begum’s mother, to ask you to reconsider your previous decision dated 19 February 2019 to deprive her daughter of her British citizenship … Ms Begum requests this reconsideration, as an act of mercy, on the basis of the following new information, namely the death of her newborn son.”

The letter says: “It is extremely unlikely that Shamima will be in a fit state to make any rational decisions.” It says the family have been unable to contact her directly and highlights that a request for help from the government was “refused in writing”.

Mohammed T Akunjee (@MohammedAkunjee) pic.twitter.com/LA0h3mzvpF

Responding to an urgent question on the subject in the House of Commons on Monday, Javid strongly rejected suggestions he was in any way responsible for the baby’s death.

He told MPs: “The death of any British child, even those children born to a foreign terrorist fighter, of course is a tragedy – but the only person responsible for the death of that child is the foreign terrorist fighter.”

His Labour counterpart, Diane Abbott, had questioned whether stripping Begum of her nationality “made it impossible for her to fulfil her duties as a mother and bring her baby home to a safe place”.

The shadow home secretary said: “Nobody on this side of the House condones what Shamima Begum did, the choices she made, the things she said ... If it was his 19-year-old daughter, however badly she had behaved, however reprehensible or near-criminal her choices, would he want her to bury three babies in the course of a year?”

A former director of public prosecutions accused Javid of moral cowardice and “treating the UK as a banana republic” in pursuit of his leadership ambitions, as the home secretary faced a chorus of outrage following the death of Jarrah.

Ken Macdonald said: “This was an abject decision by a home secretary apparently so intent on furthering his leadership ambitions that he has lost sight of sovereignty, treating the UK as a banana republic incapable of regulating its own citizens.”

The peer, who was DPP of England and Wales between 2003 and 2008 and oversaw a 2010 government review of counter-terrorism and security powers, told the Observer that Javid’s “opportunism has other costs, including a more dangerous world where stateless individuals roam with no allegiance, and the death of unprotected innocents, in this case a vulnerable British baby”.

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He warned of blowback in the UK. “No dignified self-governing state should abandon responsibility for its own citizens in this way, trying to dump them on to poorer countries with failed security arrangements. Mr Javid’s behaviour is a recipe for refugee chaos and moral cowardice of the worst sort,” he said.

The bishop of St Albans, the Rt Rev Dr Alan Smith, said of Begum: “She should come back, be properly interviewed and, if it’s found that she has broken the law, she should face the law. If it’s found that she has been radicalised, she should be given help and support.

“The home secretary has a responsibility to ensure people in this country are protected. We could have done this by taking her through due process and it is to be regretted if we are not following it, because this is a human rights issue.”