'John of the Beatles' is a prime suspect

The jihadist who beheaded US journalist James Foley is believed to be a British-born militant from London who calls himself John.

Intelligence agents in the UK and the US are looking into reports that the Islamic State fighter is the ringleader of a group of British jihadists who specialise in hostage-taking.

"John" is described as a well-educated, intelligent and highly committed member of the terrorist group.

Operating closely with two other UK-born militants, the group has been nicknamed "The Beatles" by fellow jihadists because of their nationality.

Experts last night warned that British extremists fighting in Iraq and Syria were among the most "vicious and callous" killers in the region.

Shiraz Maher, of the International Centre for the Study of Radicalisation, at King's College, London, said that more than 500 Britons who have travelled to Syria and Iraq have become more "brazen" and "arrogant" in recent months.

The beheading of Foley suggests that militants from Britain are no longer merely foot soldiers of the Islamic State but have worked their way up to senior positions within the organisation.

British police have the names of hundreds of extremists who are known to be fighting with the Islamic State and other insurgent groups.

Some have boasted on Twitter and Facebook about their involvement.