The restricted document, authored by the Ministry of Defense, MI5 and the British Special Branch, maintains al-Qaida operatives based in Britain will attempt another "spectacular" event targeting major transport facilities, such as airports and train stations, the Sunday Telegraph said.

The report said there are "some thousands" of extremists active in Britain, who are mostly British-born young men of Pakistani and Middle Eastern descent between the ages of 18 and 30. Many of them, it said, are believed to have been trained in overseas terrorist camps.

"For the foreseeable future the U.K. will continue to be a high-priority target for international terrorists aligned with al-Qaida," the government report said. "It will face a threat from British nationals, including Muslim converts, and U.K.-based foreign terrorists, as well as terrorists planning attacks from abroad."