The US has promised not to pursue the death penalty against two British Isis members accused of taking part in the beheadings of western hostages, in return for UK cooperation with the prosecution.

The pledge was given in a letter from the US attorney general, William Barr, in the case of Alexanda Kotey and El Shafee Elsheikh, members of the “Beatles” group of British Isis members, who were captured by Syrian Kurds and then handed over to US custody last October.

“The UK has received assurances from the US that if the UK agrees to transfer the evidence, the death penalty will not be sought or imposed in any prosecution of Kotey and Elsheikh, and welcomes this development,” a British official said.

British cooperation with the US prosecution had been blocked under a ruling by the UK supreme court in March, and is still in effect, UK officials pointed out.

“The government’s priority has always been to protect national security and to deliver justice for the victims and their families,” a UK home office spokesperson said. “We continue to work closely with international partners to ensure that those who have committed crimes in the name of Daesh [Isis] are brought to justice.”

According to the Defense One website, which first reported the Barr letter, the US has given the UK until 15 October to agree to its terms and share evidence through the mutual legal assistance treaty between the two countries.

In that case, all charges against the two men, who have had their UK citizenship revoked, would be covered by the guarantee on no death penalty. If that deadline was not reached, according to the report, they would be transferred to Iraq for trial, with the strong possibility of execution.

The two men were part of a group of four who are accused of being involved in the filmed beheadings of two British aid workers, David Haines and Alan Henning, as well as of two US journalists, James Foley and Steven Sotloff.

The group’s leader, Mohammed Emwazi, was killed in a US drone strike in 2015, while another, Aine Davis, was caught in Turkey and jailed for more than seven years in 2017 for membership of a terrorist organisation.

Nicholas Henin, a former French Isis hostage, welcomed the news that the death penalty would be dropped, saying on Twitter it was “great news that opens the way for a trial in the US”.