How it unfolded and how it could happen.

For the intelligence and security officials charged with the formidable task of keeping the country safe from terror attacks, it was simply a question of time before a lone fanatic brought carnage to the streets of Britain.

Politicians, the police and intelligence chiefs had been warning for months that Britain faced the imminent threat of attack following the recent wave of atrocities committed in European capitals such as Paris, Brussels, Berlin and Istanbul. And with Islamist terror groups like al Qaeda and Islamic State showing no sign of abandoning their commitment to attacking the West, Britain’s security forces have been in a state of high readiness in anticipation of such an attack.

Compared with other European cities, London had, until yesterday, escaped relatively unscathed from the growing menace posed by Islamist fanatics. Indeed, it is down to the constant vigilance of our security services that Britain has not suffered another attack on the scale of the July 7 bombings in London in 2005, in which 52 people died and more than 700 were injured.

At the time, British security officials admitted they were struggling to get on top of the growing threat posed by Islamist militants.

But lessons were learned as a result of the attacks, so that in recent years a number of high-profile terror plots have been foiled and the perpetrators brought to justice.

Consequently, UK-based Islamist fanatics have been forced to resort to carrying out lone wolf attacks, such as the murder of Fusilier Lee Rigby in Woolwich in 2013, who was hacked to death by machete-wielding terrorists.

The latest intelligence assessments suggested that this was the most likely form of attack Britain would suffer - and it was because the police and other security forces were able to respond so quickly to the attack in Westminster that the casualty toll was kept to a minimum.

Of course, there will be the inevitable inquiries into how the attack was allowed to happen, with the police and security services examining whether those responsible could have been apprehended in advance, and the dreadful loss of life avoided

Islamist terrorists are still intent on carrying out bomb attacks on civilian airliners because, if successful, they incur mass casualties.

But, as yesterday’s attack demonstrates, perpetrating acts of terror does not rely on a great amount of sophistication. All it requires is a lone fanatic determined to pursue their vile creed for innocent civilians to lose their lives.

- Telegraph Group