Mr Justice Jay said at a ruling in February that while there was a “credible suspicion that Ms Begum was recruited, transferred and then harboured for the purpose of sexual exploitation”, this did not prevent then-home secretary Sajid Javid from removing her citizenship.
Following the decision, Ms Begum’s lawyers said they would be challenging the ruling and that the legal battle was “nowhere near over”.
A three-day hearing at the Court of Appeal in London is expected to begin on Tuesday.
Ms Begum was 15 when she travelled from Bethnal Green, East London, through Turkey and into territory controlled by ISIS in 2015, before her citizenship was revoked in February 2019.
During the five-day hearing in November 2022, Ms Begum’s lawyers argued that she should have been viewed as a victim of child trafficking and that she was “persuaded, influenced and affected with her friends by a determined and effective ISIS propaganda machine”.
Why did Shamima Begum lose her UK citizenship? – video
In the ruling, Mr Justice Jay said the case had been “of great concern and difficulty”.
“This Secretary of State … maintains that national security is a weighty factor and that it would take a very strong countervailing case to outweigh it,” he continued.
“Reasonable people will profoundly disagree with the Secretary of State, but that raises wider societal and political questions which it is not the role of this commission to address.”