Al-Qaeda's deputy leader has said for the first time the group carried out the 7 July suicide bombings in London, in which 52 people were killed.
Zawahri previously stopped short of an explicit claim of responsibility
In a videotaped message aired on Arab television station al-Jazeera, Ayman al-Zawahri said al-Qaeda had the "honour" of carrying out the attacks.
Osama Bin Laden's lieutenant had previously praised the bombings and blamed them on the UK's foreign policy.
On the new tape, he also criticised Afghanistan's parliamentary elections.
He said they were not free and were carried out under US occupation.
Zawahri, shown talking to someone off-screen, said the "blessed" London attacks were targeted at "the British Crusader's arrogance and against the American Crusader's aggression on the Islamic nation for 100 years".
He denounced Britain for "the historical crime of setting up Israel and the continuing crimes in Afghanistan and Iraq".
Zawahri also criticised the British government's plans to deport the radical Jordanian cleric Abu Qatada and nine other foreign nationals said to be a threat to UK security.
Plans to toughen the UK's anti-terror laws following the bombings showed "the dreadful colonial face of Britain", he said.
In a previous tape aired on 2 September, he said the bombings were proof al-Qaeda had moved the battle to "the enemies' land".
Two other militant Islamic groups have previously claimed responsibility for the bombings.