The mother of one of four suspected Islamic State fighters has lost a legal challenge against the UK’s sharing of evidence with the US without seeking assurances he would not be executed.
El Shafee Elsheikh is accused of belonging to an IS cell which is thought to have beheaded hostages.
He is being held with another suspected cell member in northern Syria, and they may face prosecution in the US.
The court ruled the UK has no legal duty to protect Mr Elsheikh.
Britain shared 600 witness statements gathered by the Metropolitan Police with the US under a process called “mutual legal assistance” (MLA).
Mr Elsheikh’s mother, Maha Elgizouli, challenged the government’s decision.
She argued that, due to the UK’s stance on the death penalty, the government should have ensured her son would not face execution if he was extradited and tried in the US.
She also stated that it went against her own human rights, and breached data protection laws.
Lord Chief Justice Lord Burnett said: “There is no general, common law duty on Her Majesty’s government to take positive steps to protect an individual’s life from the actions of a third party and that includes requiring particular undertakings before complying with the MLA request.”
Mr Elsheikh and the other suspected IS fighter, Alexanda Kotey, were raised in the UK, but no longer have British citizenships.
The two, who are being held by Syrian-Kurdish forces, are accused of being a part of a terrorist cell known as “The Beatles” because of their British accents.
The cell are believed to have murdered foreign hostages, including Alan Henning, James Foley and David Haines; created brutal propaganda videos, and tortured dozens of people.
The other two members of the group – also from London – were Mohammed Emwazi, nicknamed “Jihadi John”, and Aine Davis.
Mohammed Emwazi was killed in a US drone strike and Aine Davis was sentenced to prison in Turkey.
All four were radicalised in the UK before travelling to Syria.
By Dominic Casciani, BBC Home Affairs correspondent
This judgement paves the way for the two Londoners to be prosecuted in the USA – after a fraught series of talks over what to do with them.
Documents in the case show that the US administration became frustrated with the UK’s refusal to take the men back while trying to tell it what to do with them if they were instead to face justice in America.
In fact, the British ambassador in Washington warned President Trump could “hold a grudge” if the UK persisted in asking for a death penalty assurance.
Ms Elgizouli’s lawyers argued that the UK’s long-standing opposition to the death penalty was therefore cast aside in the interests of political expediency.
But today’s judgement underlines that no law has been broken. Mr Elsheikh is not British – nor is he under UK control – so the obligations on ministers to act on his mother’s concerns are limited.
In the past, Britain has sought assurances from foreign governments that the death penalty would not be used in cases where the UK provided information or extradited suspects.
In this case, intelligence was shared with the US but no such assurances were sought.
However, information sharing was halted last month after Mr Elsheikh’s mother launched a legal challenge.
Home Secretary Sajid Javid said: “My priority has always been to ensure we deliver justice for the victims’ families and that the individuals suspected of these sickening crimes face prosecution as quickly as possible.
“Our longstanding opposition to the death penalty has not changed.
“Any evidence shared with the US in this case must be for the express purpose of progressing a federal prosecution.”
Ms Elgizouli’s solicitor said that she found the decision difficult to take on board.
Gareth Peirce, of Birnberg Peirce Solicitors, said that while Ms Elgizouli thinks her son should be prosecuted, she argues that it should take place within accordance of international human rights standards.
“Ms Elgizouli hopes that the opportunity will be given for the Supreme Court to consider whether it has a greater ability to explore the important factors raised in the case she has brought,” Ms Peirce said.