The Muslims mistakenly parading themselves as jihadists 'defending' Islam are plumbing new depths of horror in their vicious campaign to impose a 7th-century Islamic tyranny across large swathes of Iraq and Syria.

Indiscriminate slaughter, public beheadings and systematic starvation are all features of their murderous campaign, propped up by pernicious propaganda that inspires awe in their followers and fear in their victims.

What is particularly disturbing is the support given to the militants by many deluded British Muslims. It has been estimated that more than 500 young British men have gone to fight in the Syrian civil war.

Many of them are now moving across to Iraq to become volunteers in the Islamic State movement, which aims to resurrect a medieval-style caliphate in the region.

Only this week Reyaad Khan, a 20-year-old from Cardiff who is now a member of the Islamic State, declared on Twitter that he is preparing for 'martyrdom'. Khan boasted, in the macabre language of the jihadi lunatics, that he had 'executed many prisoners' and had witnessed 'the longest decapitation ever. And we made sure the knife was sharp.'

Alongside several other Britons, Khan has also appeared in an extremist recruitment video, urging other British Muslims to join their fanatical cause though for entirely dubious theological reasons.

As well as fighting 3,000 miles away, the jihadists have made their presence felt at home. Yesterday, several brainwashed acolytes of the infamous radical Anjem Choudary handed out incendiary leaflets to shoppers in London's Oxford Street in blind support of the Islamic State, while last weekend the black flag of jihadi fundamentalism was flown on the main gate at the entrance to a housing estate in Tower Hamlets.

What is terribly worrying is that, in the face of the IS atrocities, and extremist British Muslims' involvement in jihadism, mainstream Muslims here have remained largely silent at what is happening in Iraq today.

Where is the mass outcry against the systematic killing of the Iraqi Yazidis, the deadly harassment of Christians and the mindless destruction of their churches?

If Muslims around the world, including those in the UK, had united against jihadism in the same way that they have done against Israel, then the extremists would be far weaker.

Indeed, idiotic British Muslims l*sting after adventure might be deterred from joining IS and collaborating with injustice and immorality in the Middle East.

This deadly 'jihadism' of ill-informed religious idiots has become a terrible threat to security both at home and in Syria and Iraq.

But I am afraid we also have to face the fact that, in the Middle East, the growing strength of IS, which so attracts these recruits, is partly the direct consequence of the ill-fated and illegal war launched by George Bush and the millionaire former Prime Minister Tony Blair. There is little doubt that Saddam Hussein was a despot, but his overthrow has plunged Iraq into long-term political and social chaos.

Tragically, this is the venomous world to which so many young British Muslims are now drawn, thanks to pro-jihadi messages from some mosques, mullahs and madrassas (religious schools) which fuel and exploit their disillusion with British society.

Alienated from the liberal West, partly because of their own hardline ideology, partly because of poverty and social isolation, they do not feel they have a practical stake in modern Britain. So they fall prey to the language and imagery of the radicals, which presents jihad as both the ultimate expression of true religious devotion, and a great Boy's Own adventure.