Sunni Muslims from the UK are taking part in the conflicts "in every way", according to Shiraz Maher from King's College London's International Centre for the Study of Radicalisation.
of plotting against Muslims and interfering in their affairs before taking a knife to his victim's throat.
"Unfortunately the British participation in the conflicts now raging in both Syria and Iraq has been one of full participation, one that has seen them at the front lines, taking part in the conflict in every way.
"Unfortunately they are amongst some of the most vicious and vociferous fighters who are out there.
That is unfortunately just a part of their radicalisation."
"Unfortunately, the way the Foley video is framed, it makes it very clear now that IS (Isis) will react against any western involvement or intervention into the conflicts either in Syria or Iraq and that, of course, given that we are now helping minorities in Iraq – the Yazidis, the Kurds, for example – they regard that not just as an assault on them but they regard that ultimately as a declaration of war against Islam itself.
That will license them to attack targets and individuals and western interests as they see fit."
Maher said British and other Sunni Muslims initially went to Syria because of an "existential threat" to their faith from Shia Muslims, and their presence has since swelled Isis ranks and allowed it to increase its territory and influence.
In June, two men from Cardiff and a man from Aberdeen featured in an online recruitment video urging western Muslims to join the fighting.
Muthana's father Ahmed, whose other son, Aseel, is also in Syria, spoke out last month as part of a government-backed nationwide campaign encouraging families who suspect their children or siblings are considering going abroad to join militants to speak out. Muthana said his son had wanted to be a GP, but chose "to go with these wrong people".
"I think, 'Am I going to see him alive again?'
Maybe we won't even see the coffin – we'll just see on the news they're dead," he said.
– told the BBC last month that he had been fighting in Syria for the establishment of a caliphate.
He referred to Britain as "pure evil", and said he would only return "to raise the black flag of Islam over Downing Street, over Buckingham Palace, over Tower Bridge and over Big Ben".
They were described as "deeply religious" but reportedly ignored their family's pleas for them to come home and told them they have no intention of returning.