The sad milestone came when a lone insurgent detonated an explosive device as a foot patrol from 2nd Battalion the Parachute Regiment was returning to base on Sunday.

Mr Brown said: "My first thoughts and condolences are with the families of these soldiers, who died serving in Afghanistan with such distinction.

"I want to pay tribute to the courage of all the 100 British troops who have given their lives in Afghanistan in the service of their country. The risks they bear and the sacrifices they make should be in our thoughts, not just today but every day.

"They have paid the ultimate price, but they have achieved something of lasting value - helping turn a lawless region sheltering terrorists into an emerging democracy."

Mr Brown continued: "I do not believe democracy in Afghanistan would have survived without Nato and UN support - and British forces have been on the frontline of that international effort, and have acquitted themselves with great bravery and professionalism."

Four soldiers were injured in the attack in the Upper Sangin Valley in Helmand Province and evacuated to Camp Bastion for treatment.

One was pronounced dead on arrival and, despite medics' efforts, another two died from their wounds. The fourth soldier is expected to make a good recovery.

Next of kin have been informed and the soldiers are likely to be named later, the Ministry of Defence said.

It was the biggest single loss of life in Afghanistan since August last year, when three soldiers were killed after a US fighter aircraft dropped a bomb on them as they battled insurgents.