Speaking in English in a pronounced Yorkshire accent, the 30-year-old former teaching assistant from Leeds blamed Britain’s role in the invasion of Iraq for young, British-born Muslims being ready to attack the country of their birth. Khan said: “Until we feel secure you will be our targets. Until you stop the bombing, gassing, imprisonment and torture . . . we will not stop this fight.”

The video, broadcast on al-Jazeera television, included a claim by al-Qaeda’s second-in-command that it masterminded the synchronised bombings, which killed 56 people.

Ayman al-Zawahiri was reported to have said that the attacks on three Tube trains and a bus were a “slap for the policy of British Prime Minister Tony Blair and these operations have taken the battle to the enemy’s lands”.

Al-Zawahiri, who is Osama bin Laden’s closest confidant, did not say anything about the failed bombings on July 21.

The crucial question for police last night was where the video of Khan was filmed. He was seen in Palestinian headdress and traditional Muslim clothes. He was sitting in front of a rug or patterned wallpaper, making it impossible for police immediately to determine whether the video was made in Britain or on one of Khan’s recent trips to Pakistan.

Would-be suicide bombers usually film their video wills on the eve of their attacks. They are intended as a final message to their families, and for use as a propaganda tool to be broadcast on militant websites.

Khan appeared relaxed as he talked about the bombing of four commuter trains in Madrid in March last year and the attacks in America on September 11, 2001, which he blamed on “elected governments who perpetrate crimes against humanity”. He described the “power, wealth-obsessed agendas” of the West.

None of his accomplices was seen, and nor was there any direct message to his family. Khan was married with a small child and his wife is reported to be pregnant. His appearance confirmed his role as the ringleader of the group. He killed six people when he manually detonated his rucksack bomb on a train at Edgware Road Underground station.

The claims by al-Zawahiri are unusual. Al-Qaeda does not usually make direct claims of responsibility for terrorist operations, leaving it to sympathisers. Experts are unsure whether the footage of al-Zawahiri, an Egyptian-born doctor described as the “operational brains” behind al-Qaeda, was taken from a broadcast he made on August 4. He is wearing the same clothes and is in an identical setting with a Kala shnikov rifle beside him.

In that video he referred to the 7/7 bombings, saying: “As for the British, I tell them that (Prime Minister Tony) Blair has brought you destruction in Central London and, God willing, will bring more destruction.”

He claimed the timing for the attacks was the anniversary of Britain and other European countries ignoring a truce offer from bin Laden to withdraw troops from Iraq and Afghanistan or face a terror campaign.

David Davis, the Shadow Home Secretary, said of Khan’s broadcast: “People across Britain will be sickened by this video. Nothing can justify the murder of innocent people.”

The mother of Ciaran Cassidy, 22, who was killed on July 7, said that she never thinks about the bombers. “It’s not going to bring my son back,” Veronica Cassidy said.

Bernard Mayes, whose son James was killed, said: “Those who carried out these atrocities were brainwashed, and the people behind them need to be brought to book.”

Nazmul Hasan, 26, the uncle of Shahara Islam who died in the bus bomb, said that he was dumbfounded by Khan’s speech. “The audacity of it all is quite shocking, carrying out what he did and making the video. It’s really rubbing salt in the wound.”

Scotland Yard has always believed that the four British-born bombers — Khan, Shehzad Tanweer, Hasib Hussain and Jermaine Lindsay — would have left a final message.

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