Al-Qaeda is "on the back foot" due to the combined efforts of US-led forces in Afghanistan and the Pakistani army, British Prime Minister Gordon Brown was to say on Monday, his office said.

But in a speech strongly defending Britain's military involvement in Afghanistan, Brown was to concede that Osama bin Laden's network remains the biggest threat to Britain's national security.

"Vigilance in defence of national security will never be sacrificed to expediency... The greater international good will never be subordinated to the mood of the passing moment," he was to say, according to pre-released extracts.

"So I vigorously defend our action in Afghanistan and Pakistan because Al-Qaeda is today the biggest source of threat to our national security -- and to the security of people's lives in Britain.

"And tonight I can report that more has been planned and enacted with greater success in this one year to disable Al-Qaeda than in any year since the original invasion in 2001.

"We are in Afghanistan because we judge that if the Taliban regained power Al-Qaeda and other terrorist groups would once more have an environment in which they could operate."

The prime minister's spokesman told journalists that Brown would point to the fact that Al-Qaeda, which once operated from within Afghanistan under the protection of the Taliban, has been pushed into the border area with Pakistan.