The brutal beheading of US journalist James Foley by a Briton fighting in the ranks of Isis, which calls itself Islamic State, is the latest - and most shocking - example of British jihadists committing atrocities in Syria and Iraq.

Britain accounts for around one in four of all European fighters who have pledged their allegiance to Isis, with an estimated 500 Britons among 2,000 foreign fighters from across Europe.

One reason is the sheer ease with which people can get to Istanbul in Turkey, and then catch a bus to get into neighbouring Syria, according to Charlie Cooper, a researcher at the Quilliam Foundation. Isis wants to “show off” its foreign fighters as part of its propaganda, he added. And the unnamed man who beheaded Mr Foley “will have committed himself entirely to furthering the aims of the Islamic state" and "completely rejected his British nationality”.

The killing of the American journalist was evidence that British jihadis were "some of the most vicious and vociferous fighters" in Syria and Iraq, said Shiraz Maher, a senior researcher at the International Center for the Study of Radicalization at King's College London. They are “very much at the forefront of this conflict” with roles ranging from suicide bombers to executioners, he added.

Foreign Secretary Philip Hammond called the killing an “appalling example of the brutality of this organisation” and admitted that “significant numbers” of Britons are involved in “terrible crimes, probably in the commission of atrocities”.

In pictures: James Foley Show all 11 1 /11 In pictures: James Foley In pictures: James Foley James Foley Freelance photo-journalist James Foley (left) on the highway between the airport and the West Gate of Sirte, Libya AFP In pictures: James Foley James Foley James Foley responds to questions during an interview with The Associated Press, in Boston AP In pictures: James Foley James Foley James Foley responds to questions during an interview with The Associated Press, in Boston AP In pictures: James Foley James Foley James Foley at an airport in Tripoli (Libya), August 2011 Jonathan Pedneault via freejamesfoley.org In pictures: James Foley James Foley James Foley in Syria, 2012 Manu Brabo via freejamesfoley.org In pictures: James Foley James Foley James Foley in Aleppo, Syria, in 2012 Nicole Tung via freejamesfoley.org In pictures: James Foley James Foley James Foley rests in Syria, 2012 Manu Brabo via freejamesfoley.org In pictures: James Foley James Foley US photo-journalist James Foley resting in a room at the airport of Sirte, Libya. Jihadist group the Islamic State claim to have executed American journalist James Foley in revenge for US air strikes against its fighters in Iraq AFP In pictures: James Foley James Foley Badges in support of James Foley displayed during a panel discussion about the importance and dangers of reporting on world conflicts at a Free James Foley event on 3 May 2013 in Boston. At the time, Foley, a freelance journalist, had been held in captivity in war-torn Syria for 162 days AFP PHOTO/Don Emmert In pictures: James Foley James Foley John and Diane Foley, parents of James Foley, at a Free James Foley event on 3 May, 2013 Getty In pictures: James Foley James Foley A ribbon is tied to a tree outside the home of American freelance journalist James Foley in Rochester, N.H AP

Professor Anthony Glees, of the Centre for Security and Intelligence Studies at the University of Buckingham, told The Independent: “Why are there Brits there? In my view this is because Islamist extremist ideologies have been able to be spread with relative ease in our country under the cover of 'religion', 'free speech' and 'multiculturalism'.” He added: “A small number of British Muslims have been brainwashed by so-called preachers, from western values and convinced that they must kill to create a global caliphate.”

And they are willing to die for their beliefs.

Video: Isis recruitment video

Abdul Waheed Majeed, a 41-year-old father of three from Crawley, Sussex, died in a suicide bomb attack on a jail in Aleppo in February.

Many other Britons have been killed in the fighting. Earlier this month, 25-year-old Muhammad Hamidur Rahman, a former Primark supervisor from Portsmouth, became the latest to die, bringing the total number of Britons killed to 19.



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Meanwhile, British fighters continue to make their presence felt online. Last week, images emerged of 23-year-old Abdel-Majed Abdel Bary, from London, holding the severed head of a soldier with the caption: “Chilling’ with my homie or what’s left of him.”

Radicalised Britons, who call themselves the "Baadiya Boys" after their original base in Syria, are among the fighters who have raped and killed thousands of Yazidi refugees in northern Iraq.

Another British fighter, Reyaad Khan, 20, from Cardiff, has boasted of preparing for "martyrdom ops" and planning "fireworks" in a series of tweets over recent weeks in which he has also claimed to have “executed many prisoners”.

And last week it emerged that Nasser Muthana, 20, from Cardiff, had posted images on Twitter of a destroyed military building in northern Syria, claiming: “I’m getting good with these bombs.”