Two captured British fighters should be tried in an international criminal court, rather than sent to Guantanamo Bay, a defence minister has said.
Tobias Ellwood told two newspapers it was important that “terrorists from any origin are transparently and fairly held account for their actions”.
The fighters, Alexanda Kotey and El Shafee Elsheikh, were captured by Syrian Kurdish forces in Syria.
The BBC understands the pair have been stripped of their UK citizenship.
They were the last two of four UK IS members known as “the Beatles”, so-called because of their British accents.
The cell became infamous for its high-profile executions of Western hostages.
Mr Ellwood, who lost his brother in a terrorist attack in Bali in 2002 and who himself was caught up in the Westminster attack last year, said the captured fighters should be tried at the Hague.
In interviews in The Times and the Daily Telegraph, Mr Ellwood demanded “an agreed international process” for captured fighters.
“Guantanamo Bay created a new combatant status that bypassed the Geneva convention, used torture and failed to address a wider global jihadist insurgency that continues today,” he said.
“The horror of 9/11 meant we briefly lost sight of the standards and rule of law that took centuries to develop and fundamentally distinguish us from the terrorist.”
He added: “Given the scale of foreign fighters we should consider an agreed international process involving The Hague, which ensures terrorists from any origin are transparently and fairly held account for their actions.”
The families of some of their victims have also said the two captured fighters should face trial.
The US Department of Defence spokesman said they were “still considering options”.
“Rest assured our intention is to hold anyone accountable who commits acts like those they are alleged to have committed,” Major Adrian Rankine-Galloway from the department said.
He confirmed the captured fighters were being held by US-backed Syrian Democratic Forces (SDF) in a “detention location” in Syria.
Who were ‘The Beatles’?
Kotey, from west London, was a guard for the group that was described by US authorities as an execution cell.
The US State Department says he took part in the torture of hostages and acted as a recruiter for IS.
Elsheikh, also from London, “earned a reputation for waterboarding, mock executions, and crucifixions” while serving as the cell’s guard, the US state department says.
Both are designated terrorists by the US, which says they have used “exceptionally cruel torture methods”.
They worked with the cell’s alleged ring-leader, Mohammed Emwazi.
Dubbed Jihadi John, he was the masked militant from west London who featured in gruesome IS videos, taunting Western powers before beheading hostages.
Emwazi was killed in a drone strike in 2015 in Raqqa, the former de facto IS capital in Syria.
Aine Davis, also from west London and the fourth member of the cell, was convicted of being a senior IS member. He was jailed in Turkey last year on terrorism charges after being arrested near Istanbul in 2015.