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An educated British jihadi called John is the world’s most wanted man over the beheading of US journalist James Foley.

The killer was tonight said to be from London’s East End, and the leader of a murderous team of UK-born militants dubbed “The Beatles” by captives.

He is thought to be one of 500 British nationals fighting with the Islamic State militants in Iraq and Syria. Today US security forces launched an international manhunt to track him down.

It came after a masked man was seen brandishing a knife and ranting about America in a chilling video, before beheading photojournalist James, 40.

Militants said the horror was in revenge for US strikes against its fighters.

But this afternoon President Barack Obama vowed “justice would be done” as US officials said they were weighing up whether to send extra troops to Iraq.

John is believed to be one of the main guards of foreign hostages in Raqqa, Syria. One ex-captive described him as intelligent, educated and a devout believer in radical Islamic teachings.

He said the militant – who called himself John – and two other UK-born fighters, were referred to as “The Beatles” because of their nationality.

UK intelligence experts were reportedly comparing the killer’s voice with those of ex-Guantanamo Bay prisoners and other Brits believed to have joined the Islamic State fighters.

Prof Paul Kerswill, a linguistics expert at the University of York, said the executioner spoke in “multicultural London English” usually found in the East End.

He said: “He probably has a foreign language background but it sounds like multicultural London English, which is people from all kinds of backgrounds who mix in the East End, a new cockney.”

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British teams are examining the horrific video to “establish its authenticity” and track down the killer.

Today Prime Minister David Cameron broke off his Cornish holiday to London to chair an emergency No 10 meeting on the UK response to the “barbaric and brutal” execution.

He vowed new measures to stop more UK nationals joining IS forces, including proposals for “tougher laws to deal with terrorism and extremism”.

He said: “We have not identified the individual responsible, but it looks increasingly likely it is a British citizen.

“This is deeply shocking. But we know that far too many British citizens have travelled to Iraq and travelled to Syria to take part in extremism and violence. And what we must do is redouble all our efforts to stop people from going.”

Police have been told to stop the flying of the IS flag here and arrest anyone peddling recruitment literature for the group – once known as ISIS or ISIL.

The authorities have withdrawn 23 passports this year and made 69 arrests in a bid to stop the flow of fighters travelling to Iraq and Syria. The PM also warned of the threat to the UK from the estimated 250 Brits who have returned from Middle East battlegrounds.

He said the fight was not against the religion of Islam but “poisonous” extremists. He said: “It is a battle we have to fight in our own country using everything we have – our aid, our diplomacy, and on occasions our military powers.

“What happens in other far-flung places can come back and cause huge harm here.” But there will be no recall of Parliament, and he said it was not time for a “knee-jerk reaction”.

Foreign Secretary Philip Hammond also warned there was a risk of extremists returning home to continue their terror.

He said: “If the Islamic State, so called, becomes established in an area of Syria and Iraq, it will undoubtedly use it for a base, for launching attacks on the West.

“Equally if it gets pushed back, some will return to their countries, not just the UK, all European countries, Australia, the US, other Arab countries. We will see these people going back and potentially carrying on their fight in our homelands.” He added: “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 organisations.”

As Mr Hammond said UK troops could be sent to Baghdad to train Iraqi forces, French President Francois Hollande called for a summit to discuss ways of crushing the IS threat, while Germany and Italy said they were ready to send arms to the Iraqi Kurds battling jihadi extremists.

In the barbaric beheading video, victim James – captured in Syria in 2012 – is pictured with his head shaved and dressed in an orange jump suit, kneeling next to masked man who holds a knife to his neck. The killer then rants against President Obama and warns action against IS will result in the “bloodshed” of Americans.

President Obama said: “We will be vigilant and we will be relentless. When people harm Americans, we do what is necessary to see that justice is done. For governments and peoples across the Middle East, there has to be a common effort to extract this cancer.”

US officials last night said the Pentagon was considering sending more troops to Iraq. Mr Obama said militants had rampaged across cities, abducted women and children and subjected them to torture and rape and killed Muslims by the thousands.

He went on: “No just God would stand for what they did yesterday and what they do every single day.”

Meanwhile the Foreign Office was accused of abandoning dozens of Brits trapped by terrorist fighting in Libya.

As many as 80 UK nationals are stranded in Benghazi, which is under siege from Islamist militants.

One British woman, a mum-of-three from Cardiff, said it was “terrifying” and her house had been shelled by terrorist forces surrounding the city.

Cardiff South and Penarth MP Stephen Doughty said: “I am gravely concerned. The Foreign Office needs to do all it can to ensure there are options for them to leave.” The Labour MP added that it was “vital we don’t forget what’s happening in Libya”.

A Foreign Office spokesman said: “In the past few weeks we have actively assisted with the evacuation of more than 250 British nationals from Libya.”