Your work has helped save the lives of innocent Afghans and members of the allied forces

Tony Blair's letter

News of the letters came as it emerged British liaison officers were among several non-American personnel with US Marines at a desert airfield in southern Afghanistan, near the Taleban's final stronghold of Kandahar.

"The British have been here since the beginning. They have been very valuable," said US Marines spokesman Captain Stewart Upton.

"There are British. There are Germans. There are Australians and there are more to come," he added.

His words followed an article by US Defence Secretary Donald Rumsfeld for the News of the World in which he praised the role of British special forces in Afghanistan.

About 1,000 US Marines are now in place at the airfield near Kandahar, but there are no signs of a preparation for an assault.

'Unsung heroes'

Downing Street's decision to reveal that Mr Blair had written to Britain's spy chiefs was said to reflect his view that the intelligence agencies were the "unsung heroes" of the anti-terror campaign.

"The government and the British people are fortunate to be served by security and intelligence organisations whose professionalism is admired - and by our enemies feared - throughout the world," Mr Blair wrote.

"Without doubt your work has helped save the lives of innocent Afghans and members of the allied forces.

"Vital work has been carried out to disrupt terrorist planning in the UK, Europe and further afield," he said.

The services were said to have allowed ministers to establish Osama bin Laden's and al-Qaeda's culpability for the 11 September attacks at an early stage.

They have also provided "high quality intelligence" in the hunt for the perpetrators and have played a role informing policy through the attendance of the agency heads at War Cabinet meetings.