How to bring Islamist militants to justice, such as the so-called Isis Beatles, has sparked a diplomatic spat between the US and the UK.

The fates of Alexanda Kotey, 34, and El Shafee Elsheikh, 29, are uncertain after US Defence Secretary James Mattis ruled out their being sent to Guantanamo Bay, while their British citizenship having been stripped means they cannot return to the UK for trial.

On Tuesday (13 February), British Defence Secretary Gavin Williamson will discuss in Rome with Mattis how to deal with the foreign fighters suspected of belonging to Isis.

Kotey and Elsheikh grew up in West London and are suspected of torture and involvement in the beheading of more than 27 hostages, including Britons and Americans.

Kotey and Elsheikh would not be able to face trial in Syria as there is no adequate judicial system and the US does not recognise the International Criminal Court in the Hague.

A Whitehall source told the Times: "The Home Office find themselves in a mess. It is a bit of a pickle. I am sure that submissions are being written on how to get out of this mess. It would be very unpalatable to allow them back but they will not be the last."

Kathryn Wheelbarger, the US principal deputy assistant defence secretary, said foreign fighter detainees should return to their country of origin. About 850 Britons have joined Isis in Syria and Iraq and more than 400 have since returned and at least 100 are thought to have been killed.

Lord Carlile of Berriew, QC, a former independent reviewer of counterterrorism laws, called for the pair to be tried in the UK.

"Their victims include British nationals and their families, who have a legitimate expectation that the UK government will use its best efforts to ensure that a trial to good 'rule of law' standards will occur."