Captured jihadists Alexanda Kotey and Shafee El-Sheikh are members of the “Beatles” group of IS fighters, so-called because the quartet all possessed British accents.
The group was behind the brutal beheadings of British and US journalists and aid workers.
On Monday, the government defended its decision not to seek assurances Kotey and El-Sheikh won’t be executed if they face justice in America, despite a growing backlash – including from among Conservative MPs.
Ministers have been accused of breaching the UK’s long-standing opposition to the death penalty, drawing anger from human rights groups.
The controversy has put Home Secretary Sajid Javid under scrutiny, after the decision was revealed in a letter, sent last month from Mr Javid to US attorney general Jeff Sessions, which was obtained by the Daily Telegraph.
However, answering an urgent question in the House of Commons, security minister Ben Wallace insisted government policy on the death penalty has not changed.
Mr Javid did not answer MPs’ questions as he was visiting Durham Police as part of a cabinet awayday in the North East.
In his place, Mr Wallace revealed Prime Minister Theresa May was aware of, and agreed with, the joint Home Office and Foreign Office decision not to seek US assurances.
Mr Wallace, despite not commenting directly on the case, also effectively confirmed previous reports Kotey and El-Sheikh were stripped of their UK citizenship after joining IS.
Labour’s shadow home secretary Diane Abbott called on the government to reverse its decision to “abandon our principled opposition to the death penalty”, an act she branded “abhorrent and shameful”.
She told Mr Wallace it was not possible to be “a little bit in favour of the death penalty”, adding: “Either we offer consistent opposition or we don’t.”
Ms Abbott also noted how Diane Foley – the mother of murdered US journalist James Foley – had voiced her opposition to the death penalty.
But, Mr Wallace highlighted how Ms Foley also “said she thought it was right they faced justice in US courts”.
“Who are we to deny that to those victims in the US if the UK holds some of that evidence that may make it possible?,” he asked.
He later told MPs: “I also am aware as an ex-soldier that all states including states that oppose the death penalty use lethal force when it has to do so to keep itself secure.
“We risk being seen as hypocrites if we say on the one hand we will not ever make an exception for assurances but on the other hand we will use lethal force in battlefields to kill people without due process.”
Mr Wallace also insisted the government’s opposition to Guantanamo Bay has not altered, despite the Daily Telegraph reporting other documents show the UK will not formally oppose Kotey and El-Sheikh being sent to the US detention facility without trial.
Among Tories expressing unhappiness at the government’s decision, former attorney general Dominic Grieve told MPs it was a “major departure from normal policy”.
Labour’s Yvette Cooper, chair of the House of Commons’ home affairs committee, accused Mr Javid of having “unilaterally ripped up” the UK’s principle on the death penalty “on a Friday afternoon in summer”.
However, Conservative backbencher Andrew Percy claimed Britonsc would be behind the government’s decision as he urged ministers to ignore “unrepresentative grandstanding” from critics and continue “assisting the US in prosecuting these murderous terrorist scum”.
In his letter to Mr Sessions, dated 22 June, Mr Javid said: “I am of the view that there are strong reasons for not requiring a death penalty assurance in this specific case, so no such assurances will be sought.
“I have instructed my officials to set out the terms of our assistance and to work with your officials to action the request.
“As you are aware, it is the long held position of the UK to seek death penalty assurances, and our decision in this case does not reflect a change in our policy on assistance in US death penalty cases generally, nor the UK government’s stance on the global abolition of the death penalty.”
Amnesty International branded the decision “a huge backward step” in efforts to abolish the death penalty around the world, with Mr Javid having now left the UK “open to charges of hypocrisy and double standards”.
A third member of “the Beatles” death squad, Mohammed Emwazi – who became known as “Jihadi John” – was killed by a drone strike in Syria in 2015.
The fourth, Aine Lesley Davis, is in a Turkish jail.