The former schoolgirl from London, who left the UK for Syria when she was just 15, had lived under Daesh rule for more than three years before she was discovered in a refugee camp last February.
Sajid Javid removed her British citizenship shortly afterwards, and the government has used the same powers against dozens of alleged Daesh members to prevent their return to the UK.
The decision was appealed by Begum’s lawyers, accusing the government of making her stateless and exposing her to the risk of death or inhuman and degrading treatment.
On Thursday, the Court of Appeal granted Begum permission to launch a judiciary review against that decision.
“Ms Begum should be allowed to come to the United Kingdom to pursue her appeal albeit subject to such controls as the Secretary of State deems appropriate,” a summary of the decision said.
The Home Office moved to immediately block the effect of what it called a “very disappointing decision by the court”.
“We will now apply for permission to appeal this judgment, and to stay its effects pending any onward appeal,” a spokesperson said.
In the full judgment, Lord Justice Flaux said national security concerns over Begum “could be addressed and managed” in the UK.
“If the Security Service and the Director of Public Prosecutions consider that the evidence and public interest tests for a prosecution for terrorist offences are met, she could be arrested and charged upon her arrival in the UK and remanded in custody pending trial,” he added.
Shamima Begum: 'I would like them to re-evaluate my case with a bit more mercy'
“If that were not feasible, she could be made the subject of a Terrorism Prevention and Investigation Measures (TPIM) order.”
In a judgment agreed by Lord Justice Singh and Lady Justice King, Lord Justice Flaux said that “fairness and justice” outweighed national security concerns in the case.
He added: “The only way in which she can have a fair and effective appeal is to be permitted to come into the United Kingdom to pursue her appeal ... it does not follow that because she could not have a fair and effective appeal, her appeal should be allowed.”
Tom Hickman QC, representing Begum, previously told the court she had no “fair and effective means of challenging the decision to deprive her of her British citizenship”.
“The only things that are clear are that Shamima Begum was a child when she left the UK and had been influenced to do so,” he added.
Sir James Eadie QC, representing the Home Office, said the fact that Begum could not fully engage in the appeal procedure was “a result of her decision to leave the UK, travel to Syria against Foreign and Commonwealth Office advice and align with Isis”.
Begum will be allowed to appeal both the decisions of the SIAC and the High Court.
SIAC had ruled that the decision to revoke her British citizenship did not render her stateless and was lawful, as she was a “citizen of Bangladesh by descent”.
However in February 2019, Bangladesh’s ministry of foreign affairs said she was not a citizen and there was “no question“ of allowing her into the country.
A statement said Begum had never visited the country or applied for dual nationality, and that the government was “deeply concerned” over the British government’s claims.
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.”
The British government removed citizenship from 139 people “for the public good” between January 2016 and December 2018.
A 2016 report commissioned by the government warned that removing extremists’ citizenship left them free to continue terrorist activities abroad, prevented monitoring and encouraged the “dangerous delusion that terrorism can be made into a foreign problem”.
In August last year, former defence minister Tobias Ellwood told The Independent the detention of thousands of jihadis and their families in Syria was creating conditions for an Isis resurgence.
“We’ll see Daesh 2.0,” he warned. “We’ll see a repeat of al-Qaeda regrouping and becoming a very real threat, and that threat won’t just pose itself in the Middle East, but also to Britain.”