The analysis will be carried out by members of Scotland Yard's anti-terrorist branch and the Special Branch, MI5 and experts from GCHQ. Details will be passed on to the US and allied security agencies.
The one sure thing the film appears to show is that Khan and his accomplices were not "duped" into carrying out the attacks, as had been suggested, but had set out to kill themselves and fellow residents of the UK.
The video also appears to undermine conspiracy theories prevalent among the Muslim community that the London attacks were somehow orchestrated by British/American/Israeli intelligence services.
The Labour MP Shahid Malik said: " I believe there is a hardcore rump within the British Muslim community that didn't actually believe, somehow, that Sidique and his cohorts were responsible. Rampant conspiracy theories mushroomed out of control."
The Arabic satellite television channel, al-Jazeera, said the tape was delivered to its headquarters in Qatar through channels normally used by al-Qa'ida. It bears the "al-Sahab" or "the clouds" logo used as a signature by al-Qa'ida.
Despite that and the presence of Ayman al-Zawahiri on the tape, security sources believe the original tape was doctored to include Osama bin Laden's deputy. Al-Qa'ida's suicide bombers are not known to make such videos.
Khan, in his message, praises "heroes like our beloved Sheikh Osama bin Laden, Dr al-Zawahiri and al-Zarqawi [ the Jordanian-born insurgent leader in Iraq]". There is no evidence that he was in direct contact with them. However some observers insisted last night that the tape showed a direct al-Qa'ida connection.
Abdel Bari-Atwan, editor of London-based Arabic newspaper Al Quds, said the message indicated the link between the bombings and policy on Iraq. He said: "It does confirm that al-Qa'ida managed to penetrate some support of Muslim extremists in this country."
Others, however, were dismissive of the scenario. Robert Emerson, a security analyst, said: "It would have been a huge propaganda coup for al-Zawahiri to be side by side with Khan in a genuine video. They now appear, instead, to be exploiting what had been done by a cell of sympathisers, but not people they actively controlled."
Al-Zawahiri, in his message, says the attacks were partly in response to the West's failure to take up an offer of a "truce" by Bin Laden in which the al-Qa'ida leader promised to stop attacks against countries not involved in Iraq and Afghanistan and actively opposed Israel. There is no mention of this in Khan's speech.
Paul Cornish, head of the international security programme at Chatham House, said the tape showed anger over UK foreign policy had fed into the bombers' motivation. He said analysts would now be "pulling to bits" the tape to examine it in minute detail.Reuse content