The government is facing a legal challenge over its refusal to ensure two Isis militants are not executed when they are sent to the US for prosecution.

Elsheikh’s mother is attempting to launch a judicial review of Sajid Javid’s unprecedented decision not to seek assurances that the death penalty would not be used, when he handed over British intelligence.

But the legal action may not stop the pair, who are alleged members of the British terror cell known as “the Beatles”, being transferred from custody in Syria to the US.

“The Americans can do what the Americans want,” a government source told The Independent.

A spokesperson for the US State Department said it would “continue to pursue all diplomatic avenues” but would not confirm whether its plans for Kotey and Elsheikh were affected by the pause in British assistance.

After the home secretary’s letter to the US attorney general was leaked, a solicitor representing Elsheikh’s mother wrote to the home secretary “asking for his immediate undertaking that mutual assistance should not continue to be provided”.

Security minister Ben Wallace: two Isis terrorists would 'roam around UK' if government did not share intelligence with US

“The letter explained that in the absence of an undertaking, an immediate application would be made to court for an injunction preventing further provision of assistance in light of an unlawful decision by the minister, until such time as a court could determine its legality,” said a statement from solicitors’ firm Birnberg Peirce.

“Mr Elsheikh’s mother had long made clear her wholesale opposition to the actions of Isis. Her request is that the norms of internationally accepted due process form the basis of any trial of accusations concerning her son.”

The Home Office pledged to stop any further assistance to US prosecutors until a court determines whether the legal action will progress, and must formally respond to the mother’s claim within a week.

But British authorities may have already sent over the bulk of material from a four-year counterterror investigation into Kotey and Elsheikh to their American counterparts, who may be able to progress regardless of the challenge.

A Home Office spokesperson confirmed it had agreed to a “short-term pause” in mutual legal assistance because of the mother’s request, but added: “The government remains committed to bringing these people to justice and we are confident we have acted in full accordance of the law and within the government’s longstanding MLA policy.”

Solicitors said their application for judicial review aimed to determine the legality of Mr Javid’s failure to demand assurances that Kotey and Elsheikh would not be subjected to the death penalty.

“The application raises questions of enormous constitutional importance, including the ability of a minister – without reference to parliament – to agree to so complete a departure from a prohibition understood by all to have the status of constitutional certainty and without any public debate,” representatives said.

Sajid Javid said there were ‘strong reasons’ not to ask for assurances against the death penalty (PA)

It called a letter sent by the home secretary to the US attorney general on 22 June a “clear and dramatic departure from the UK’s long standing international and domestic commitment” to oppose the death penalty.

The document, which only became public when it was leaked to the media this week, said Britain “does not currently intend to request, nor actively encourage” the transfer of Kotey and Elsheikh to Britain from Syria, where they are currently being held by the Syrian Democratic Forces (SDF) alongside thousands of other foreign fighters.

“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,” Mr Javid added.

“We believe that a successful federal prosecution in the US is more likely to be possible because of differences in your statute book and the restrictions on challenges to the route by which defendants appear in US courts.”

The Howard League for Penal Reform is also examining the possibility of legal action against the government, and MPs of all parties condemned the failure to consult parliament.

Tory grandee Ken Clarke was among the Conservatives condemning the government, saying the UK “must not let its standards slip” in the face of terror.

Ben Emmerson QC, a former United Nations special rapporteur on human rights and counterterrorism, said the decision was “unprincipled, incompetent and almost certainly unlawful”, and Lord Carlile called it “extraordinary”.

Diane Foley, the mother of murdered hostage James Foley, has called for Kotey and Elsheikh to be jailed rather than killed – and their former captive Nicolas Henin told The Independent he did not want them to be given the “satisfaction” of martyrdom.

Timeline: The emergence of Isis Show all 40 1 /40 Timeline: The emergence of Isis Timeline: The emergence of Isis 2000 Abu Musab al-Zarqawi (pictured here) forms an al-Qaeda splinter group in Iraq, al-Qa’eda in Iraq. Its brutality from the beginning alienates Iraqis and many al-Qaeda leaders. Timeline: The emergence of Isis 2006 Al-Zarqawi is killed in a U.S. strike. Al-Zarqawi’s successor, Abu Ayyub al-Masri, announces the creation of the Islamic State in Iraq (ISI). Reuters Timeline: The emergence of Isis 2009 Still al-Qaeda-linked ISI claims responsibility for suicide bombings that killed 155 in Baghdad, as well as attacks in August and October killing 240, as President Obama announces troop withdrawal from Iraq in March. Getty Images Timeline: The emergence of Isis 2010 Abu Bakr al-Baghdadi becomes head of ISI, at lowest ebb of Islamist militancy in Iraq, which sees last U.S. combat brigade depart. Timeline: The emergence of Isis 2012 In Syria, protests (pictured here starting in Daree) have morphed into what president Assad labelled a “real war” with emergence of a coalition of forces opposed to Assad’s regime. Syria group Jabhat al-Nusra are among rebel groups who refuse to join, denouncing it as a “conspiracy”. Bombings targeting Shia areas, killing more than 500 people, spark fears of new sectarian conflict. Sunni Muslims stage protests across country against what they see as increasingly marginalisation by Shia-led government. AP Timeline: The emergence of Isis 2013 Al-Baghdadi renames ISI as the Islamic State in Iraq and Syria, or Isis, as the group absorbs Syrian al-Nusra, gaining a foothold in Syria. In response, al-Qaeda chief Ayman al-Zawahiri (Bin Laden’s successor) concerned about Isis’ expansion orders that Isis be dissolved and ISI operations should be confined to Iraq. This order is rejected by al-Baghdadi. AFP Timeline: The emergence of Isis 2014 - January Isis fighters capture the Iraqi cities of Fallujah and Ramadi, giving them base to launch slew of attacks further south. AP Timeline: The emergence of Isis 2014 - June Isis declares itself the Caliphate, calling itself Islamic State (IS). The group captures Mosul, Iraq’s second largest city; Tal Afar, just 93 miles from Syrian border; and the central Iraqi city of Tikrit. These advances sent shockwaves around the world. Timeline: The emergence of Isis 2014 - June Around the same time Isis releases a video calling for western Muslims to join the Caliphate and fight, prompting new evaluations of extremists groups social media understanding. Timeline: The emergence of Isis 2014 - June Isis take Baiji oil fields in Iraq - giving them access to huge amounts of possible revenue. EPA Timeline: The emergence of Isis 2014 - August James Foley is executed by the group as concerns grow for second American prisoner, fellow reporter Steven Sotloff. AP Timeline: The emergence of Isis 2014 - August Obama authorises U.S. airstrikes in Iraq, helping to stall Isis’ along with action by Kurdish forces following the deaths of hundreds of Yazidi people on Mount Sinjar. Timeline: The emergence of Isis 2014 - September Isis release video showing Steven Sotloff’s murder prompting Western speculation his executioner is same man who killed Mr Foley. EPA Timeline: The emergence of Isis 2014 - September Obama tells us that America “will hunt down terrorists who threaten our country” EPA Timeline: The emergence of Isis 2014 - September Isis release a video appearing to show David Haines, who was captured by militants in Syria in 2013, wearing an orange jumpsuit and kneeling in the desert while he reads a pre-prepared script. It later shows what appears to be the aid worker's body. Rex Timeline: The emergence of Isis 2014 - September Peshmerga fighters scrabble to hold positions in the Diyala province (a gateway to Baghdad) as Isis fighters continue to advance on Iraqi capital. AFP Timeline: The emergence of Isis 2014 - October Aid worker Alan Henning is killed. Self-imposed media blackout refuses to show images of him in final moments, instead focuses upon humanitarian care. AP Timeline: The emergence of Isis 2014 - October Isis raise their flag in Kobani, which had been strongly defended by Kurdish troops. The victory goes against hopeful western analysis Isis had overextended itself, while alienating much of the Muslim population through the murder of Henning. Victory causes fresh waves of Kurdish refugees arriving in Turkey. Timeline: The emergence of Isis 2014 - November American hostage, who embarced values of Islam, Peter Kassig and 14 Syrian soldiers are shown meeting the same fate as other captives. But intelligence agencies will be poring over the apparently significant discrepancies between this and previous films. Seramedig.org.uk Timeline: The emergence of Isis 2015 - February Isis has released a video revealing the murder by burning to death of a Jordanian pilot held by the group since the end of December 2014. Reuters Timeline: The emergence of Isis 2015 - February Isis militants have released videos which appear to show the beheading of Japanese hostages Haruna Yukawa and Kenji Goto. Timeline: The emergence of Isis 2015 - February American aid worker, Kayla Mueller was the last American hostage known to be held by Isis. She died, according to her captors, in an airstrike by the Jordanian air force on the city of Raqqa in Syria, though US authorities disputed this. AP Timeline: The emergence of Isis 2015 - February Isis militants have posted a gruesome video online in which they force 21 Egyptian Coptic Christian hostages to kneel on a beach in Libya before beheading them. Egypt vowed to avenge the beheading and launched air strikes on Isis positions. AP Timeline: The emergence of Isis 2015 - February The British Isis militant suspected of appearing in videos showing the beheading of Western hostages has been named in reports as Mohammed Emwazi from London. Rex Features Timeline: The emergence of Isis 2015 - March Isis triple suicide attack has killed more than 100 worshippers and hundreds of others were injured after the group members targeted two mosques in the Yemeni capital of Sanaa. AP Timeline: The emergence of Isis 2015 - April Iraqi forces have claimed victory over Isis in battle for Tikrit and raised the flag in the city. EPA/STR Timeline: The emergence of Isis 2015 - April Isis has claimed responsibility for a suicide bomb attack in Afghanistan that killed at least 35 people queuing to collect their wages and injured 100 more. EPA Timeline: The emergence of Isis 2015 - April Isis’ media arm released a 29-minute video purporting to show militants executing Ethiopian Christians captives. The footage bore the extremist group’s al-Furqan media logo and showed the destruction of churches and desecration of religious symbols. A masked fighter made a statement threatening Christians who did not convert to Islam or pay a special tax. Timeline: The emergence of Isis 2015 - May Abu Bakr al-Baghdadi, the leader of Isis has been "incapacitated" by a spinal injuries sustained in a US air strike in Iraq. He is being treated in a hideout by two doctors from Isis’ stronghold of Mosul who are said to be "strong ideological supporters of the group". Timeline: The emergence of Isis 2015 - May Isis has also claimed responsibility for killing 300 of Yazidi captives, including women, children and elderly people in Iraq AP Timeline: The emergence of Isis 2015 - May Isis attack on Prophet Mohamed cartoon contest in Texas was its first action on US soil. Two gunmen were shot and killed after launching the attack at the exhibition. Elton Simpson and Nadir Soofi have been named as the attackers at the Curtis Culwell Centre arena in Garland. Timeline: The emergence of Isis 2015 - May Isis’s deputy leader, Abu Alaa Afri, a former physics teacher who was thought to have taken charge of the deadly terrorist group, has been killed in a US-led coalition airstrike. Timeline: The emergence of Isis 2015 - May US special forces have killed a senior Isis leader named as Abu Sayyaf in an operation aiming to capture him and his wife in Syria. Getty Images Timeline: The emergence of Isis 2015 - May Iran-backed militias are sent to Ramadi by the Iraqi government to fight Isis militants who completed their capture of the city. Government soldiers and civilians were reportedly massacred by extremists as they took control and the army fled. Charred bodies were left littering the city streets as troops clung on to trucks speeding away from the city. Ramadi is the latest government stronghold to fall to the so-called Islamic State, despite air strikes by a US-led international coalition aiming to stop its advance in Iraq and Syria. AFP Timeline: The emergence of Isis 2015 - May Isis rounded up civilians trapped in Palmyra and forced them to watch 20 people being executed in the historic city’s ancient amphitheatre. The Unesco World Heritage site was overrun by militants, threatening the future of 2,000 year-old monuments and ruins. Thousands of Palmyra’s residents fled but many are still living within the city walls, while the UN human rights office in Geneva said it had received reports of Syrian government forces preventing people from leaving until they retreated from the city. Getty Timeline: The emergence of Isis 2015 - May A group of Isis-affiliated fighters have captured a key airport in central Libya. The militants took control of the al-Qardabiya airbase in Sirte after a local militia tasked with defending the facility withdrew from their positions. Affiliates of Isis, already control large parts of Sirte, the birthplace of former Libyan leader Muammar Gaddafi and a former stronghold of his supporters. Timeline: The emergence of Isis 2015 - June The US Air Force has destroyed an Isis stronghold after an extremist let slip their location on social media. According the Air Force Times, General Herbert "Hawk" Carlisle, commander of Air Combat Command, said that Airmen at Hulburt Field, Florida, used images shared by jihadists to track the location of their headquarters before destroying it in an airstrike. Reuters Timeline: The emergence of Isis 2015 - June Kurdish forces captured a key military base in a significant victory in Raqqa as well as town of Tell Abyad. YPG fighters, backed by US-led airstrikes and other rebels, consolidated their gains, when they seized the key town on the Syria-Turkey border. They are now just 30 miles to the north of Raqqa and have cut off a major supply route deep inside Isis-held territory. Ahmet Silk/Getty Timeline: The emergence of Isis 2015 - June Isis has released gruesome footage claiming to show the murder of more than a dozen men by drowning, decapitation and using a rocket-propelled grenade as it seeks to boost morale among its fanatical supporters. Timeline: The emergence of Isis 2015 - June Isis has begun carrying out its threat to destroy structures in the ancient Syrian city of Palmyra, blowing up at least two monuments at the Unesco-protected site as Syrian government troops made advances on the Islamist’s positions. AFP

But Downing Street threw its weight behind the Home Office by saying Theresa May supported its handling of the case.

Ben Wallace, the security minister, argued in the House of Commons that seeking an assurance against the death penalty might “get in the way” of a US trial.

He was jeered for suggesting the alternative was letting Kotey and Elsheikh “roam free”, and used Isis’s horrendous crimes to bat away questions from MPs over why the government had not followed normal policy.

“We are not going to seek assurances because it is our reality that we do not think we have the evidence here to try them in the UK,” he added, confirming they had been stripped of British citizenship.

“At the end of the day, this is about the security of our country and about security being delivered where it can be done.”

Terror experts have cautioned that executing Kotey and Elsheikh would hand Isis a fresh propaganda victory as it seeks to regroup in new “wilayat” (provinces) following territorial losses across Syria and Iraq.

Originally from London, the pair were declared “Specially Designated Global Terrorists” by the US State Department ahead of their capture in January, with official documents naming them as members of the “the Beatles” and saying the cell had beheaded more than 27 hostages and tortured many more.

Surviving captives have told of their brutality, which included torture, waterboarding, electric shocks, mock executions and crucifixions.