Shamima Begum, who fled the UK and joined the Islamic State group, was smuggled into Syria by an intelligence agent for Canada.
Files seen by the BBC show he claimed to have shared Ms Begum’s passport details with Canada, and smuggled other Britons to fight for IS.
Ms Begum’s lawyers are challenging the removal of her citizenship, arguing she was a trafficking victim.
Canada and the UK declined to comment on security issues.
Ms Begum was 15 when she and two other east London schoolgirls – Kadiza Sultana, 16, and 15-year-old Amira Abase – travelled to Syria to join the terrorist group IS in 2015.
At the main Istanbul bus station, the girls met Mohammed Al Rasheed, who would facilitate their journey to IS-controlled Syria.
A senior intelligence officer, at an agency which is part of the global coalition against IS, has confirmed to the BBC that Rasheed was providing information to Canadian intelligence while smuggling people to IS.
The BBC has obtained a dossier on Rasheed that…