LONDON -- The voice of an apparent British militant narrating the video showing the beheading of American journalist James Foley has triggered renewed questions about why the U.K. is a breeding ground for jihadis.

At least 400 Britons are among the estimated 2,000 Europeans who are fighting for the Islamic State of Iraq and al-Sham (ISIS), according to Prime Minister David Cameron. And the ease with which Europeans can travel into Syria through Turkey has alarmed intelligence officials in the West.

On Thursday, NBC News reported that three militants with British accents had been dubbed "The Beatles" by hostages taken in Syria. A person close to several recent hostage negotiations said "The Beatles" were harsher than other guards. "Whenever the Beatles showed up, there was some kind of physical beating or torture," the source added.

Britain has a “deeply entrenched problem,” according to the Quillam Foundation, an anti-extremist think tank. “London historically has had Islamist ideology being taught openly without being challenged and there are many people who have grown up knowing and believing that the only way to be Muslims is to create this Islamic state," said Harris Rafiq, Quilliam's head of outreach. "It's not surprising that jihadis have been able to cherry-pick these people."

The true number of British jihadis could be even higher. Khalid Mahmood, a U.K. parliament lawmaker from Birmingham, England, estimates that at least 1,500 Brits have been recruited by extremists fighting in Iraq and Syria over the last three years — more than double the number of Muslims currently serving in the U.K. military.

However, the numbers are inevitably higher in European nations with large Muslim populations. “When you look at the raw numbers, it’s not the best way to get a sense of how deep the problem is,” said Shiraz Maher, senior research fellow at the International Centre for the Study of Radicalization at London’s King’s College. “What we’ve done is to wade through the numbers of foreign fighters in relations to the Muslim population of those countries. When you do it like that, Belgium is actually way off the chart. But the Scandinavian countries feature very highly, and Britain as well.”

"Islamist extremist ideologies have spread with relative ease under the cover of 'free speech' "

Britain’s problem with radicalized Muslims — described by Foreign Secretary Philip Hammond as “a poison, a cancer” - is made acute chiefly because of its role as the biggest global ally of the United States in the tarnished invasions of Iraq and Afghanistan.

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As the same time, English is gaining traction as the language of choice for recruitment videos and other online propaganda because it has greater viral potential on social media.

“It’s no coincidence that the [Foley beheading] video was in English,” said Ghaffar Hussain, managing director of Quillam. "If the West, particularly America, is where you are trying to get your message heard, it makes sense.”

Social media is a powerful tool, especially for recruiting young male Muslims, according to Hussain. “The violent messages appeal to the macho element and the sense of going to join a fight,” he said, citing the recent case of Muhammad Hamidur Rahman, a 25-year-old killed while fighting in Syria after quitting his job at a British branch of fashion chain Primark. “One minute you’re working as a shop assistant, next minute you’re on the front line with a gun. It’s an attractive idea for many.”

Last month, engineering student Abdul Raqib Amin also told how he had left Aberdeen, Scotland, to join ISIS. He described the journey to join the militants as one of the happiest moments of his life.

YouTube, Twitter and Facebook have partly replaced Britain’s previous funnel for extremism — radical preachers invited to speak at hardline mosques and Islamic schools.

A recent ICSR report also noted that social media had become essential in spreading the extremist message from the fighting fronts of wars that are so dangerous that Western journalists struggle to cover them. “Social media is no longer virtual: it has become an essential facet of what happens on the ground,” the report said.

Since the 2005 bombings on London's transport network, tackling fanatical messages has become an obsession in Britain - a mostly secular country despite Queen Elizabeth’s formal position as “defender of the faith” and head of the Church of England. Such has been the country’s multicultural ethos that the heir to the throne, Prince Charles, once stated that, as king, he wanted to be “defender of all the faiths,” including Islam.

Wearing the Islamic veil, or burqa, is banned in public places in France, Belgium and parts of Switzerland. In contrast, England’s professional body for family attorneys recently began offering training courses in drafting wills that are compliant with Islamic Shariah law.

Some commentators argue that this tolerance has left Britain more exposed to the threat posed to its Muslim communities by radical Islam.

“In the U.K., with our proud tradition of freedom and not wanting to get involved in religious disputes, we have been bending over backwards to regard murderous ideologies as expressions of free speech,” said Anthony Glees, director of the University of Buckingham’s Centre for Security and Intelligence Studies. “Islamist extremist ideologies have spread with relative ease under the cover of ‘free speech’ and ‘multiculturalism’.”

Glees added that Britain’s predominantly Sunni Muslim population was also particularly vulnerable to the allure of the jihadi message from Sunni ISIS.

“Across Europe, Muslim populations come from different parts of the Islamic world. British Muslims come primarily from Pakistan, Bangladesh and the Horn of Africa — in other words, places where the political culture is virulent,” he said. “I think it is a particular problem in the U.K. in as much as most of our Muslims are from Sunni background. It is not that that you are more likely to be a violent extremist if you’re from a Sunni background but it is much easier to call yourself a preacher. There is no hierarchy in the Sunni religion. Anyone can say they’re a preacher, and this gives them an opening into mosques, the college lecture circuit and other places where young people congregate.”

The problem has continued despite a series of multi-million dollar government campaigns to prevent young British Muslims from becoming radicalized. It is easy to find Muslims in London who publicly defend the establishment of the Islamic State and who favor Shariah law over the law of the land.

Quilliam's Harris fears the problem could come home to roost, saying he would be horrified if an attack on London occurred — "but I would not be surprised."

ISIS leader Abu Bakr al-Baghdadi "wants to make a statement he is a bigger terrorist, a bigger jihadi, a bigger figure than Osama Bin Laden was, and he has got to try to undertake an attack on the West. And London is a prime target for that display," Harris added.

Michele Neubert and Sarah Burke of NBC News contributed to this report.