In the tape, the origins of which have been unconfirmed, a man appearing to be Mohammad Sidique Khan said he was driven to his actions in response to the “atrocities” committed against Muslims.
The tape was broadcast by Arab television network Al-Jazeera.
Speaking in English with a northern English accent, the man outlined his motivations for taking part in the attack.
The taped message was accompanied by a separate message by al-Qaeda’s second in command Ayman al-Zawahri.
“I and thousands like me have forsaken everything for what we believe in. Our driving motivation does not come from tangible commodities that this world has to offer. Our religion is Islam,” said Khan.
Looking relaxed but gesticulating with a pen as he spoke, Khan was filmed wearing a turban and a waterproof jacket and against the background of a coloured tapestry.
“Your democratically elected governments continue to commit atrocities against my people over the world.
“Their support makes you directly responsible just as I am directly responsible for protecting and avenging my Muslim bothers and sisters,” he said, in an apparent message to Western countries.
“Until we feel security you will be our targets. Until you stop the bombing the gassing, the imprisonment and torture of my people we will not stop this fight.
“We are at war and I am a soldier, now you too will face the reality of this situation,” he said.
The suicide bombings on London’s transportation network on July 7 killed 56 people and injured hundreds.
Zawahri, in his message, warned there would be more attacks like the London suicide bombings on Western countries that took part in the US-led wars on Iraq and Afghanistan.
Zawahri said the July 7 attacks on London’s transport system were a “slap in the face” for British Prime Minister Tony Blair, adding that they had “moved our battle right to the enemy’s doorstep”.
The tape appeared to be an extract from a videotape already partly aired by al-Jazeera on August 4, in which Zawahri warned Britain and the United States of more attacks.
British police said they would look into the videotape.
A spokeswoman for London’s Metropolitan Police said: “We are aware of the tape, we will consider it as part of our ongoing investigation.
British Prime Minister Tony Blair’s office refused to comment on the video.