Terror alerts, 9/11-style bombings and murders of British citizens will soon come to London’s streets, according to chilling threats from UK citizens fighting alongside Islam’s most violent terrorist group operating in Syria and Iraq.

The threat comes from British nationals fighting for the Sunni militant group calling themselves the Islamic State of Iraq and the Levant (ISIL/ISIS) in Syria. According to the Sunday Times, they promise that after they’re done there, Britain is next. The message comes from three such fighters, all youngsters in their teens and twenties.

According to the Times, these aren’t regular disenchanted youths at all: one is a 20-year-old hacker from Birmingham, who once stole Tony Blair’s details and posted them online, later serving time for an unrelated violence charge.

The “black flag of jihad” will fly over Downing Street, Junaid Hussain warned on June 4, spelling out the horrors to come. He’s been fighting in Syria for over a year now.

Another, 19-year-old Muhammad Hassan, from Portsmouth, was a student at a prestigious school. He warned on Twitter that if the US doesn’t cease threats over drone strikes on ISIS positions, 9/11-style attacks on America would follow.

The third, also from Portsmouth, promised a “killing spree” of British citizens if he were ever to return to Britain.

“Imagine if someone were to detonate a bomb at voting stations or ambushed the vans that carry the casted [sic] votes. It would mess the whole system up,” came another tweet from Junaid Hussain, before he re-tweeted a warning from a like-minded countryman for British people to “watch out,” because “we’ll come back to the UK and wreak havoc.”

Among his other offenses, Hussain has also been seen posting bomb-making advice on the internet, as well as tips on how to smuggle explosive devices through airport security.

Getting to Syria from Britain isn’t as difficult as some would think: the youths simply travel to Turkey, then crossed the border. One of them was caught by CCTV cameras at the time, traveling on a holiday flight from London Gatwick airport to Turkey’s Antalya. Hassan’s group (not including the aforementioned tweeters) had three other people, all in their mid-twenties. After reaching Turkey, they headed over to Syria by car.

This is nowhere near the first time the public has seen British nationals fighting alongside Middle-Eastern extremists. There have been numerous threats, government monitoring of scores of returnees and reports that British Sunni jihadists use Syria as a training ground before taking the battle to home soil.

What’s more striking is how news of their particular cruelty is widespread in Syria itself: Britons comprise an overwhelming majority of foreigners inside ISIS, an organization who are labeled ‘terrorists’ for their bloody campaign of ruthless executions and body mutilations even by other elements of the Syrian opposition.

One Free Syrian Army General spoke in horror of what some 400 British extremists fighting there were capable of doing. He pointed out that ISIS’s already dark record did not originate from actions by local fighters alone.

Further illustrating the scale of foreign involvement in ISIS operations, Mike Rogers, the US chairman of the Intelligence Committee, speaking to Fox News alleged that there are “thousands” of American and European volunteers and mercenaries fighting for ISIS, which “has the capability to tap people with Western passports to send them back to Europe and the United States for terrorist activity.”

“This is as dangerous as it gets,” said the Republican politician, who urged Barack Obama to use his influence with Sunni Arab states, such as Saudi Arabia and Egypt, to rein in ISIS – which is likely partly funded by powerful sympathizers within those countries.

The current threats take place amid a fearsome escalation of violence in Iraq, which has been spiraling out of control the closer ISIS got to Baghdad. Army desertions abound as militants storm through the country, capturing hundreds of millions of dollars in American weapons and equipment and parading them in videos.

The rebels are currently storming through the major cities of Mosul and Tikrit, continuing to post videos of beheadings on their way to the capital.

Britain has been taking steps to curb the spread of extremism on its soil. Over the weekend, Twitter blocked all ISIS-related accounts and the accounts of its supporters at home in the UK. This is a sensible move, given most of the communication between worldwide supporters of the jihadist cause is done on social networks, where the above British-born terrorists all use new Islamic names.

Prime Minister David Cameron is also ringing alarm bells, asserting that Britain has become too tolerant for its own good, bringing about a rise in extremism. The PM has called on several occasions for the protection and promotion of “British values” as a counterweight to the perceived malignant Islamization of Britain. He now believes the government has been too soft in a number of ways, according to a recent public address.

“It isn’t enough simply to respect these values in schools. They’re not options, they’re the core of what it is to live in Britain,” Cameron told the public.

"In recent years we have been in danger of sending out a worrying message: that if you don't want to believe in democracy, that's fine; that if equality isn't your bag, don't worry about it; that if you're completely intolerant of others, we will still tolerate you,” the PM asserted.

“That's not just led to division; it has also allowed extremism – of both the violent and non-violent kind - to flourish. We need to be far more muscular in promoting British values and the institutions that uphold them,” he said, shortly after a major scare involving an alleged Trojan Horse plot to Islamicize certain British schools, which prompted an investigation and much bickering within the government.