"Until we feel security, you'll be our target," he said. "Until you stop the bombing, gassing, imprisonment and torture of my people, we'll not stop this fight. We are at war, and I am a soldier. Now you too will taste the reality of this situation."
Most al-Qa'ida tapes and videos, however threatening, are given an other-worldly quality by their misty, translated rhetoric. Khan's message, by contrast, is blunt and direct. Wearing a keffiya scarf around his head, but otherwise clad in Western dress, the presumed leader of the London bombers says: "I'm going to keep this short and to the point, because it's all been said before by far more eloquent people than me."
Broadcast exactly eight weeks after Khan and his accomplices bombed three Tube trains and a bus in the capital, killing themselves and 52 travellers, the posthumous diatribe was a jolt to Londoners, just as they were beginning to breathe a little more freely. The video, passed to the al-Jazeera network, was spliced with a statement by al-Qa'ida's deputy leader, Ayman al-Zawahiri, who calls the "blessed London battle" a "sip from the glass that the Muslims have been drinking from".
The Egyptian physician, Osama bin Laden's right-hand man and al-Qa'ida's chief ideologue, seeks to turn Britons against the Government, saying: "Blair not only disregards the millions of people in Iraq and Afghanistan, but he does not care about you as he sends you to the inferno in Iraq and exposes you to death in your land because of his crusader war against Islam."
Security sources have been quick to point out that Zawahiri does not refer directly to Khan. Nor does the bomber take unambiguous responsibility for the 7 July attacks. But this weekend, as the tape is analysed for both overt and covert meanings, an unofficial intelligence contact said: "It is all rather embarrassing. This gives the appearance that al-Qa'ida did mastermind the bombings, while officially the security forces are saying there is no evidence and, worse still, no arrests or identification of a 'mastermind'.
"It will be hard to persuade the public that both groups [the suicide bombers of 7 July and those accused of the failed 21 July attack] were not part of a wider network, that might go to the top of the al-Qa'ida leadership in Pakistan."
Although it makes clear that the suicide bombers were not dupes, as some had speculated, the video raises many more questions than it answers. From Khan's appearance, friends believe it was filmed up to a year ago, possibly during one of the two visits he made to Pakistan before the bombings. "If Khan recorded the video in Pakistan, then that is pretty scary," said Robert Ayers, a homeland security expert at the Royal Institute of International Affairs.
Others take the view, however, that if the video had been made in Pakistan, Khan would have been brandishing an automatic rifle in authentic jihadi style. Rather than having received his orders in Pakistan and trained for the attack there, it is more likely that Khan recorded his statement in this country, according to this view. After the bombings, the tape would have been passed from hand to hand until it reached al-Qa'ida, which then added Zawahiri's retrospective blessing.
"It is noticeable that Khan does not specify any details about what he is going to do, which suggests it was done well before the bombing," Mr Ayers said. "If I had to bet - and this is pure speculation - I would say Khan's message was recorded in Yorkshire a few months ago."
Evan Kohlmann, a Washington-based consultant on terrorism to the US government, was in no doubt, however, that the video demonstrated al-Qa'ida's involvement. "It shows there was an al-Qa'ida mastermind behind this, and that it was not just the work of local cells," he said. "At least in the case of the 7 July cell, it is clear that they went to Pakistan, got training, met al-Qa'ida leaders and were tasked with the terror mission.
"I am surprised that some security officials are saying there is no proof of al-Qa'ida's involvement. The video was put together by the same production house that did the 9/11 hijackers' statements. This is classic al-Qa'ida methodology."
A Whitehall source believed the delay in releasing the video contradicts this theory. "If al-Qa'ida had the Khan footage all along, it is hard to believe that they would not have released it at the time of Zawahiri's last video, at the beginning of August," said the source, but Mr Ayers disagreed. "Immediately after the bombings they didn't need any more publicity," he said. "Releasing it two months on is very effective."
But whatever the history of the video, its appearance emphasises how far the investigation has yet to go in determining the motives of the bombers, what help, if any, they received from the outside, and whether the 7 July group had any links to the perpetrators of the abortive attack two weeks later. This is despite Khan's known links to other terror suspects, and his identification by a jailed al-Qa'ida operative in the US.
Worst of all is the fear that the terrorist's posthumous message could inspire other groups, as yet unidentified, to emulate him and his accomplices. "He is speaking as a global jihadist," said Dr Rohan Gunaratna, a leading expert on al-Qa'ida. "For him there is not one conflict or one movement. He is a spokesman for a new type of jihad which will affect Europe in coming years, even if Bin Laden or Zawahiri are killed or captured."
For all the descriptions of al-Qa'ida as a movement no longer capable of planning and ordering attacks, it remains a powerful propaganda force, as a Whitehall source admitted, saying: "It is a fantastic use of the media to push some very political messages."
The video immediately pushed the Government into yet more denials that Iraq was inflaming Muslim militancy against Britain, despite Khan honouring al-Qa'ida's principal ally in Iraq, Abu Musab al-Zarqawi. Jack Straw, the Foreign Secretary, repeated the argument that Iraq was merely the latest excuse for terrorist attacks which had begun well before the country was invaded.
But in an interview to be screened today by GMTV, Sir Crispin Tickell, a former British ambassador to the UN, said: "We were one of the principal performers in Iraq, we are one of the principal victims of these terrorist attacks, so to say that it's got nothing to do with it is, frankly, moonshine." Another well-informed source with links to the intelligence services said: "You cannot avoid the conclusion that Iraq is a machine generating Islamist radicals ... The Government continues to deny it, but it is pretty clear that Iraq has handed them [the radicals] a fantastic advantage."
UK bomber's chilling words
I'm going to keep this short and to the point because it's all been said before. And our words have no impact upon you, therefore I'm going to talk to you in a language that you understand. Our words are dead until we give them life with our blood...
I and thousands like me are forsaking everything for what we believe. Our driving motivation doesn't come from tangible commodities that this world has to offer.
Our religion is Islam - obedience to the one true God, Allah, and following the footsteps of the final prophet and messenger Mohamed...
Your democratically elected governments perpetuate atrocities against my people all over the world. And your support of them makes you directly responsible, just as I am directly responsible for protecting and avenging my Muslim brothers and sisters ...
Untilyou stop the bombing, gassing, imprisonment and torture of my people we will not stop this fight. We are at war and I am a soldier...
I myself I make dua [pray] to Allah ... to raise me among those whom I love, like the prophets, the messengers, the martyrs and today's heroes, like our beloved Sheikh Osama bin Laden, Dr Ayman al-Zawahiri and Abu Musab al-Zarqawi...