The US has told the UK that two Islamic State suspects will not face the death penalty if convicted of the killings of Western hostages in Iraq and Syria.
Alexanda Kotey and El Shafee Elsheikh are accused of being the last two members of an IS cell dubbed "The Beatles" because of their UK accents.
The US sought the UK's help in the case but a legal fight over the use of the death penalty has stymied co-operation.
The US has now made clear the two will not be executed if found guilty.
In a letter to Home Secretary Priti Patel, US Attorney General William Barr said the US authorities would not seek the death penalty against the two men and "if imposed, it will not be carried out".
In the light of the assurances, he said he hoped the UK would share "important evidence" about the men promptly.
"If we receive the requested evidence and attendant cooperation from the UK, we intend to proceed with a United States prosecution," he wrote.
"Indeed, it is these unique circumstances that have led me to provide the assurance offered in this letter."
A Home Office spokesman said the UK "continue to work closely with international partners to ensure that those who have committed crimes in the name of Daesh are brought to justice".
The pair, who are in US military custody in Iraq, were British citizens, but have been stripped of their UK nationality.
They are alleged to have been members of an IS kidnap gang behind the killings of a number of Western hostages, including American journalists and British aid workers, in Iraq and Syria in 2014.
The victims were beheaded and their deaths filmed and broadcast on social media.
The UK believes the men cannot be legally extradited to the UK, but in 2018 it emerged that the US was preparing the ground to prosecute the men – and that it had asked the UK for information that would help convict them.
In response, ministers said they would share intelligence, without opposing a death penalty sentence.
But co-operation with the US was halted after the mother of El Shafee Elsheikh launched a legal challenge, arguing the UK's position was in breach of its internationally recognised opposition to capital punishment.
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.