A Canadian spy smuggled the British schoolgirl Shamima Begum and two friends into Syria to join ISIS and his role was covered up by police and the British Government, a new book has claimed.
Ms Begum, who became a jihadi bride in Syria, was later branded a "threat to national security" and stripped of her UK citizenship by the home secretary, a move backed by the Supreme Court.
Since the breakup of the so-called Isis caliphate, Ms Begum has been living in a Syrian detention camp and is now planning to use the allegation of an official cover-up in her forthcoming appeal against the decision to remove her British nationality.
She will argue that as the child victim of people traffickers she should be protected by the government, not persecuted.
Ms Begum left her east London home and travelled to Turkey with two friends - Kadiza Sultana and Amira Abase - on 17 February 2015.
According to The Secret History Of The Five Eyes, a book about Britain and its international intelligence allies, the girls were met in Istanbul and smuggled to IS terrorists in Syria by Mohammed Al Rasheed, an alleged double agent working for Isis and Canada, one of the UK's closest partners.
The book claims Al Rasheed was arrested in Turkey days later and his alleged role in the trafficking of the three girls emerged, though it wasn't immediately made public.
If so, it means that Scotland Yard was making public appeals for information about the girls' whereabouts while an agent working for a key ally already knew what had happened to them.
Two officials from the Canadian Security Intelligence Service visited Scotland Yard in early March and admitted their agent had helped Ms Begum and her friends, though it wasn't aware until after they had reached IS territory, the book claims.
The Begum family's lawyer, Tasnime Akunjee, said that three weeks after the Canadian's admission he was still being "fobbed off" by Scotland Yard and told it had no news.
He told Sky News: "We were informed about a Canadian asset's involvement in the trafficking of Shamima for some time way back in 2015, but we've never had any confirmation from any other official source.
"The problem is I'm dealing with the UK police force who have lied and the Canadian security service who are supposed to be our allies that made a calculation to allow children to be trafficked to their demise. There's not a lot of trust going on here."
Scotland Yard said: "We do not comment on matters relating to intelligence."
A UK government spokesman said: "It is our longstanding policy that we do not comment on operational intelligence or security matters."
The Canadian Security Intelligence Service has been contacted for comment.
Last summer, during an interview, Ms Begum said she wanted to be brought back to the UK to face charges and added in a direct appeal to the prime minister that she could be an "asset" in the fight against terror.
She insisted she had been "groomed" to flee to Syria as a "dumb" and impressionable child.
Mr Akunjee said in an earlier statement: "In November, Shamima Begum will have a hearing in the SIAC [Special Immigration Appeals Commission] court, where one of the main arguments will be that when former Home Secretary Sajid Javid stripped Shamima Begum of her citizenship, leaving her in Syria, he did not consider that she was a victim of trafficking.
"The UK has international obligations as to how we view a trafficked person and what culpability we prescribe to them for their actions.
"The most shocking element of this is that integral to that traffic network were the actions of a Canadian intelligence asset. Someone who is supposed to be an ally protecting our people rather than trafficking British children into a war zone. Intelligence gathering looks to have been prioritised over the lives of children.
"Britain has lauded its efforts to stop Isis and the grooming of our children by spending millions of pounds on the Prevent programme and online monitoring.
"However, at the very same time, we have been cooperating with a Western ally, trading sensitive intelligence with them whilst they have effectively been nabbing British children and trafficking them across the Syrian border for delivery to Isis, all in the name of intelligence gathering.
"The calculation here is that the lives of British children, and the risk of their death, is part of the algorithm of acceptable risk that our Western allies have taken."