The killing of a soldier in broad daylight yesterday in Woolwich, London, has all the hallmarks of the very nightmare MI5 has feared - low-key fanatics arming themselves with basic weapons and targeting a soldier or other victim at random.
The apparent attackers yesterday, believed to be Islamic militants armed with knives including a meat cleaver, were shot and injured by police.
British police and security services have had repeated success in foiling plots since the July 7 attacks in 2005, which left 52 innocent people dead, but they have been under constant threat of the lone wolf terrorist.
Dubbed the "Nike terrorists" after the brand's slogan "Just Do It", Britain has faced a new generation of Islamic extremists nearly impossible to detect.
They are self-starting fanatics who have radicalised themselves over the internet and, while many may be inspired by al-Qaeda, they operate without command or control from the terror group.
MI5 was urgently trawling through its systems to see if the alleged assailants in Woolwich were known to them. The new threat's nature is that such attacks can come out of nowhere, with no network or obvious plot for MI5 or the police to detect in advance.
In 2010, Roshonara Choudhry was convicted of trying to murder MP Stephen Timms with a knife at his constituency office after having watched extremist material on the internet.
This year, a group of Birmingham terrorists who turned up too late for their target, a rally by the English Defence League, were picked up on by the security services only after a traffic officer caught them driving home without insurance. The group had knives, guns and viable home-made bombs.
Senior Whitehall sources have regularly expressed relief that random, basic but deadly attacks have not been more commonplace in Britain.
The threat of self-starters increased dramatically with the emergence of Anwar al-Awlaki, an al-Qaeda preacher based in Yemen. He gave sermons in English over the internet and encouraged followers anywhere to launch whatever attack they could, spawning the "Just Do It" motto.
Awlaki was killed in 2010 but almost every fanatic convicted of terror plots in Britain in recent years has taken inspiration from him.
Last year, Jonathan Evans, who was then director-general of MI5, warned that one of the biggest threats was from "lone actors".
John O'Connor, a former Flying Squad commander at Scotland Yard, said: "This has all the hallmarks of a very low-key terrorist incident which raises a number of problems for the authorities.
"This type of attack is very difficult to protect against because it is not as though you are talking about a network of people following their plans. This raises the whole ball game.
"The bottom line of it is that this could spring up anywhere and that's the concern. It's very difficult to keep a tab on where this is going and where the threat level is."
He added: "It seems to me that this is a departure from the established type of attacks that you see or the established plans that you see of terrorism causing mass murder.
"You're into a new round of terror threats in this country, particularly as you don't know the full extent of it."
The security services and police have foiled more than a dozen terror plots on British soil since 2005, including a major plot in every year since.