British extremists are among the most "most vicious and vociferous fighters" in the Islamic State's (IS) ranks in Syria and Iraq, a jihadism expert has said after a man with an English accent was filmed beheading US journalist James Foley.
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, including as suicide bombers and executioners.
The black-clothed and masked man, who speaks with traces of a London accent, threatened bloodshed in the gruesome video, speaking with confidence of when he accuses America of plotting against Muslims and interfering in their affairs, before taking a knife to his victim's throat.
If it is confirmed, as it appears, that he is British he will be seen as the most extreme example yet of a fighter travelling from the UK to take part in brutal violence as part of the militant IS movement.
Mr Maher, a senior research fellow at ICSR, told BBC Radio 4's Today Programme this morning: "Unfortunately the British participation in the conflicts now raging in both Syria and Iraq has been has been one of full participation, one that has seen them at the front lines, taking part in the conflict in every way.
"So we have seen British fighters out there operating as suicide bombers, we have seen them operating as executioners.
"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."
The video apparently shows James Foley's beheading by a British IS militant
Maher warned that the self-proclaimed caliphate was likely to carry out more atrocities because of Western efforts to help people in the region.
He told Today: "Unfortunately, the way the Foley video is framed, it makes it very clear now that IS 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.
"And therefore, that is the sort of narrative we have, of course, heard from al Qaida in the past. That will license them to attack targets and individuals and western interests as they see fit."
Story continues beneath slideshow
Mr Maher said British and other Sunni initially went to Syria because of an "existential threat" to their faith from Shia Muslims, and their presence has since swelled IS's 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 with the Islamic State in Iraq and Greater Syria (Isis) group, now known as IS.
Nasser Muthana, 20, Reyaad Khan, 20, and Abdul Raqib Amin, 26, appeared in the video and are thought to be among more than 400 Britons fighting in Syria and Iraq.
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.
Mr 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.
Another man, calling himself Abu Osama, with an accent suggesting he comes from the north of England, told the BBC last month that he had been fighting in Syria for the establishment of a caliphate, an Islamic state.
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".
Manchester twin sisters Salma and Zahra Halane are thought have travelled to Syria in June, possibly to join an older brother who had become a fighter there.
Barrister Carl Gardner pointed out on Twitter that any UK citizen can be prosecuted for a murder committed abroad.
A British citizen can be prosecuted in our courts for a murder committed abroad: http://t.co/jl2afWESOj
— Carl Gardner (@carlgardner) August 19, 2014
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.
More than 100 imams have called on British Muslims not to travel to Syria and Iraq, writing an open letter urging local communities "to continue the generous and tireless effort to support all of those affected by the crisis in Syria and unfolding events in Iraq", but to do so "from the UK in a safe and responsible way".
Philip Hammond said 'significant numbers' of Britons had been involved in 'terrible crimes'
Reacting to the video of Foley's beheading, Foreign Secretary Philip Hammond said the intelligence agencies were tracking and monitoring Britons who could be involved with extremist groups in Syria and Iraq.
He acknowledged that "significant numbers" of British nationals had been involved in "terrible crimes" and "probably in the commission of atrocities".
He said: "We are very much aware of the threat from foreign nationals operating in Syria and Iraq, that's not just to the UK it's to all Western countries and other Arab countries as well.
"This is a poison, a cancer, what's going on in Iraq and Syria and it risks spreading to other parts of the international community and affecting us all directly."
He added: "This may come as news to some people but it certainly does not come as news to us.
"We are absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with IS and other extremist organisations.
"This is something we have been tracking and dealing with for many, many months, I don't think this video changes anything it just heightens awareness of a situation which is very grave and which we have been working on for many months."
In 2002, British-born Omar Saeed Sheikh was convicted in Pakistan of killing the American journalist Daniel Pearl, whose beheading was also filmed.
Though he initially confessed, Sheikh is appealing his conviction after Khalid Sheikh Mohammed, who is currently detained in Guantanamo, apparently confessed to the murder.