US Edition THE SAS is hunting for Osama bin Laden in the Indian state of Kashmir after intelligence reports stated that he had sought the protection of an extremist Islamic group.
The SAS soldiers involved are part of a joint 40-man operation with Delta Force, the US equivalent of the SAS.
The decision to send in British Special Forces followed the Prime Minister's visit to India, Pakistan and Afghanistan last month.
Mr Blair's trip, made amid fears of a nuclear conflict between India and Pakistan over Kashmir, was followed almost immediately by a visit to both countries by Colin Powell, US secretary of state.
At about that time, Indian intelligence told the CIA that they believed bin Laden was hiding in the Himalayan mountains in Kashmir, protected by the Islamic guerrilla group Harkat-ul-Mujahideen.
The group, whose sphere of operations sprawls across Afghanistan, Pakistan and India, is believed to have smuggled him into one of many remote areas that are nearly impossible for the Indian army to police.
Harkat-ul-Mujahideen is the latest incarnation of a militant Islamist group which is extremely closely linked to al-Qa'eda and has kidnapped a number of westerners, including two Britons who are still missing.
The hunt is employing a range of high-tech devices. A spy satellite above the Indian Ocean operated jointly by American and British signals intelligence is being used to monitor any communications between bin Laden and other members of al-Qa'eda.
Other satellites capable of using infra-red imaging to detect the movement of humans in the snow of the remote Himalayan passes are also looking for the terrorist leader.
A senior defence source who recently returned from the region said the SAS troopers were "acting in an advisory role" for Indian Army special forces.
Amid fears that any military intervention might ignite the border conflict between India and Pakistan, the SAS has been given strict orders to stay clear of any firefights and merely collect intelligence.
But Pakistan's announcement this week that members of its Inter-Service Intelligence (ISI) had been ordered to disband its section aiding Kashmiri separatist groups including Harkat-ul-Mujahideen appears to indicate that it has been informed of the SAS-Delta Force operation.
One source said the team was mounting "one of the most technical covert operations of the war" to pinpoint any activity by members of the separatist group.
"The whole area is ultra sensitive," he said. "But bin Laden has a history here with some of the terror groups and he may have regarded it as a safe haven.
"He knows we are not going to start bombing the area or sending in the marines, but there are lots of other things we can do and if he is alive he is definitely not safe."
Bin Laden has not been seen since shortly before US and British Special Forces entered the Tora Bora complex in Afghanistan in November.
The source refused to comment on what would happen if they managed to find bin Laden in Kashmir.
But he added that the Indian Army was "as keen as the rest of the world" to see the al-Qa'eda leader dead.
Many of Harkat-ul-Mujahideen's fighters trained in the Afghanistan terror camps. It recruits young Muslims from Pakistan and Britain and has received significant financial support from bin Laden and the ISI.
The group was originally called Harkat-ul-Ansar (Movement of the Volunteers) and was sponsored by America in the 1980s to fight against the Soviet occupation of Afghanistan. It later turned its attention to the Kashmir issue.
It was declared an international terrorist organisation by the US in 1987 and subsequently merged with an existing group, Harkat-ul-Mujahideen (Movement of the Religious Fighters).
As Harkat-ul-Ansar, it was responsible for the kidnapping of two Britons in 1995 - Paul Wells, 23, from Blackburn, and Keith Mangan, 33, from Middlesbrough - two Americans, a Norwegian and a German.
One of the Americans escaped. The Norwegian was found beheaded. Nothing is known of what happened to the others.
The kidnappers were demanding the release of three Kashmiri separatist leaders, one of whom was the British-born Islamic militant Ahmed Omar Saeed Sheikh, who is currently held by the Pakistani authorities in connection with the murder of the US journalist Daniel Pearl.