The two suspected IS "Beatles" should be brought back to the UK and tried, according to a former counter-terrorism regulator.
Lord Carlile, an ex-independent reviewer of terrorism legislation, said he was vehemently opposed to Alexanda Kotey and El Shafee Elsheikh being sent to Guantanamo Bay.
He told the Observer that a British trial was the "proper forum" for justice - with the fate of the pair undecided.
It comes as Justice Secretary David Gauke told Peston on Sunday the government was still considering what its options are.
Kotey and Elsheikh, who along with Mohammed Emwazi - also known as Jihadi John - and Aine Davis allegedly beheaded a number of foreign prisoners while fighting for so-called Islamic State, were captured last month in Syria.
Kotey, from west London, was heading to Turkey and planned to travel back to Europe when he was detained, according to the Syrian Democratic Forces.
The decision where to send Kotey and Elsheikh, currently detained by Kurdish militia fighters, continues to be debated.
It has been rumoured they could end up in Guantanamo Bay or the International Criminal Court at The Hague.
There are concerns that a transfer to The Hague would cause too many delays.
Lord Carlile said the that radicalised Britons should be "returned to the country of origin" and tried "in a normal way with British rights, British duties, British obligations and British responsibilities.
"If people are tried properly, as they would be in the British courts, it would show that the UK is taking a very serious approach to deradicalisation but also to dealing with extremism," he said.
Urging against Guantanamo, he said: "I would expect the Foreign Secretary to urge the Americans and the Syrians to accept that British justice is a compliant and efficient system and that the most convenient forum and indeed the proper forum for such cases is the home country."
Mr Gauke said on Sunday: "At this stage we are considering what the options might be.
"As someone who believes in our principles of the rule of law and so on, my belief is that they need to be properly brought to justice. But, I think we need to consider what are the practical options in these circumstances."
Emwazi, Davis, Kotey and Elsheikh were dubbed "The Beatles" because of their English accents.
The US Department of Defence previously said they were "still considering options" with regards to the two men's future.