On the eve of the first anniversary of the co-ordinated attacks that killed 52 people, the footage showed a bearded man, believed to be Shehzad Tanweer from the city of Leeds in northern England.
"What you have witnessed now is only the beginning of a string of attacks that will continue and become stronger until you pull your forces out of Afghanistan and Iraq," he says in the video.
It was filmed before he boarded a train on the London Underground's Circle line and detonated the bomb that killed himself and six others.
British police said they were aware of the tape.
Andy Hayman, assistant commissioner of the Metropolitan police in London, said: "There can be no doubt that the release of the video at this time can only cause maximum hurt and distress to the families and friends of those who died [on July 7]."
The tape also contained footage of Ayman al-Zawahiri, the number two to Osama bin Laden in al-Qaeda, praising Tanweer.
Ayman al-Zawahiri praised
"He (Tanweer) studied physical fitness at university and he had a passion for boxing," al-Zawahiri was shown as saying. "Even though he was from a well-off family, his clothes and appearance didn't show that."
Tanweer and al-Zawahiri did not appear together on the tape and they had different backgrounds when speaking.