The BBC reports that it "has located an Islamist website that has published a 200-word statement issued by an organisation saying it carried out the London bombings." Obviously, nobody currently knows the validity of the claim. The BBC notes that "The website has previously carried statements purporting to be from al-Qaeda."
The statement by the otherwise unknown Secret Organization Group of al-Qaeda says, "Nation of Islam and Arab nation: Rejoice for it is time to take revenge against the British Zionist Crusader government in retaliation for the massacres Britain is committing in Iraq and Afghanistan. The heroic mujahideen have carried out a blessed raid in London. Britain is now burning with fear, terror and panic in its northern, southern, eastern, and western quarters."
The statement also claims that "We continue to warn the governments of Denmark and Italy and all the Crusader governments that they will be punished in the same way if they do not withdraw their troops from Iraq and Afghanistan. He who warns is excused."