The US and UK are in talks about what to do with two men from west London who were detained in Syria last month on suspicion of terrorist activity, the home secretary has said.
Amber Rudd said the captured men, Allexanda Kotey and El Shafee Elsheikh, should go on trial.
They are suspected of being members of an Islamic State cell that killed Western hostages.
The pair have been stripped of their UK citizenship.
Where should the IS 'Beatles' be tried?
The men are accused of being the last two of four UK IS members known as "the Beatles", so called because of their British accents.
It has been suggested they could be sent to the US military prison at Guantanamo Bay.
Alternatively, they could be put on criminal trial in the US or at the International Criminal Court, The Hague, which can prosecute people for genocide, war crimes, crimes against humanity and aggression.
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 Kotey and Elsheikh should be tried at The Hague, reports BBC.