The mother of a suspected Islamic State terrorist from the UK will start a legal challenge over the Government’s refusal to ensure her son and another suspect will not face the death penalty in the US.

Alexandra Kotey and El Shafee Elsheikh are said to have been members of a brutal four-man cell of IS executioners in Syria and Iraq, nicknamed The Beatles because of their British accents, who were responsible for killing a number of high-profile Western captives.

The pair were captured in January, sparking a row over whether they should be returned to the UK for trial or face justice in another jurisdiction.

Elsheikh’s mother Maha Elgizouli is bringing a case at the High Court, challenging Home Secretary Sajid Javid’s decision to share intelligence with US authorities under a “mutual legal assistance” (MLA) agreement – without seeking assurances the men would not face execution if they were extradited and tried there.

Her case will be heard by the Lord Chief Justice Lord Burnett and Mr Justice Garnham on Monday and Tuesday.

Mr Javid faced intense criticism over his decision after a letter he wrote to US Attorney General Jeff Sessions was leaked, with MPs accusing him of breaching the UK’s long-standing opposition to the death penalty.

The Government’s former reviewer of anti-terror legislation Lord Carlile branded the move “extraordinary” and said it was a “dramatic change of policy”.

After Ms Elgizouli launched legal action, the Home Office announced in July that it had agreed to a “short-term pause” of the MLA process with the US.

Lawyers from Birnberg Peirce Solicitors, who are representing Ms Elgizouli, said the case will explore whether the “claimed rationale” for the Home Secretary’s decision was “profoundly flawed”.

They said in a statement: “The claimant’s case is that it is neither a lawful nor a legal exercise of power to afford assistance when it facilitates the imposition of the death penalty in the US.

“The claim raises questions of enormous constitutional importance, including the ability of a minister without reference to Parliament to agree to a complete departure from a prohibition understood by all to have the status of constitutional certainty and without any public debate.

“The minister’s letter revealed a clear and dramatic departure from the UK’s longstanding international and domestic commitment to oppose the continuing exercise of the death penalty.”

Kotey and Elsheikh are said to have been members of the cell which also included Mohammed Emwazi – known as “Jihadi John” – who was killed in a US air strike in 2015, and Aine Davis, who has been jailed in Turkey.

Emwazi appeared in a number of videos in which hostages, including British aid workers David Haines and Alan Henning and US journalists James Foley and Steven Sotloff, were killed.

Mr Foley’s mother Diane said she was opposed to the death penalty.

Prime Minister Theresa May supported Mr Javid’s original decision, which was also backed by Boris Johnson when he was foreign secretary.

But the MLA sparked widespread criticism in Westminster from across the political divide.

A number of Tories raised concerns, with former attorney general Dominic Grieve warning the decision represented a “major departure from normal policy”.

Shadow attorney general Baroness Chakrabarti said Mr Javid appeared to have “secretly and unilaterally abandoned Britain’s opposition to the death penalty”.