The power has become a major part of the UK’s efforts to prevent jihadis returning from Syria, by depriving them of travel documents and any right to assistance or repatriation.
Isis members are believed to make up a significant proportion of at least 150 people who have had their British citizenship removed for the “public good” since 2014.
A legal challenge was brought by a woman who is being held in the same Syrian camp as Shamima Begum, and can only be identified as D4.
Mr Justice Chamberlain ruled that the decision to remove her British citizenship was “void and of no effect“ because she was not notified, and that she remains a UK national.
The judgment found the government had gone beyond its legal powers by removing a requirement to tell people who are being stripped of their nationality.
“This case is not about whether there were good reasons to make this rule. It is about whether parliament gave the home secretary the power to make it,” said a judgment given on Friday.
“As a matter of ordinary language, you do not ‘give’ someone ‘notice’ of something by putting the notice in your desk drawer and locking it. No-one who understands English would regard that purely private act as a way of ‘giving notice’.”
The ruling could spark other legal challenges by alleged Isis members who were subject to the same procedure, and could force the Home Office to change its processes.
The High Court heard that D4 has been detained in the al-Roj camp since January 2019 alongside other women and children caught while leaving Isis territory.
In December that year, a minister removed her UK citizenship but she was not formally informed until October 2020, after her solicitors asked the government to repatriate her and were refused.
D4 then appealed to the Special Immigration Appeals Commission and started judicial review proceedings in the High Court.
The law states that the government ”must give the person written notice“ of a citizenship deprivation decision, providing reasons for it and notifying them of their right to appeal.
In 2018, the Home Office changed regulations setting out how notice can be given if someone's whereabouts are unknown, there is no address to send documents to and they do not have a lawyer.
Under the new rules, notice of a decision to revoke someone's citizenship was ”deemed to have been given“ to the person in question if the Home Office made a record of it and put it on their file.
Mr Justice Chamberlain said the British Nationality Act 1981 did not give the Home Office the power to “dispense with the requirement to give notice”.
He declared the rule change “ultra vires”, meaning it was outside the government’s legal power, and added: “Parliament did not give the home secretary power to make regulations that treat notice as having been given to the person affected when it has not been given to that person but instead has simply been placed on a Home Office file.”
The judge called citizenship deprivation a “very substantial interference” with fundamental rights and said the government either has to give people notice or change the law.
“There are, no doubt, arguments in favour of amending the statutory scheme so as to permit the making of an order without giving notice in a case where it is not reasonably possible to do so, but the proper place for those arguments is parliament,” he added.
Mr Justice Chamberlain made a declaration that the removal of D4's citizenship was void, but suspended the ruling’s effect to allow the Home Office time to consider whether to appeal against his ruling.
The use of citizenship deprivation for the “public good” has rocketed since 2014, with 148 people subject to the measure by the end of 2018.
The government has not released more recent statistics, and the figure does not include D4, Begum and high-profile jihadis whose citizenship was removed from 2019 onwards.
A report by the Independent Reviewer of Terrorism Legislation, Jonathan Hall QC, said the deprivation of citizenship “has been a major part of the United Kingdom's response to those who have travelled to Da'esh-controlled areas”.
Mr Hall has warned of an “inadequate level of independent review” of the power and its effects and asked for it to be brought into his remit, but the request was refused by the government.
A government-commissioned review warned in 2016 that removing extremists’ citizenship left them free to continue terrorist activity abroad, prevented monitoring and encouraged the “dangerous delusion that terrorism can be made into a foreign problem”.
It quoted research saying that the measure amounted to “a policy of catch and release, setting up today’s convicts as tomorrow’s foreign fighters” and exporting them to places where they could not be monitored.
The former defence minister Tobias Ellwood previously told The Independent that the British government might be creating the conditions for an Isis resurgence by leaving fighters in Syrian prisons.
“We’ll see Daesh 2.0,” he warned. “They will regroup to fight another day.”
The Kurdish authorities holding D4, Begum and numerous alleged Isis members from Britain have repeatedly called for the UK to repatriate them.
Of more than 900 people who travelled to Syria and Iraq from the UK, an estimated half have returned but only around 40 were successfully prosecuted.
A Home Office spokesperson said: “The government will always take the strongest possible action to protect our national security and the priority remains the safety of our citizens.
“We are carefully considering the implications of this ruling, including an application to appeal.”