Gordon Brown paid tribute today to the 100 British troops killed in Afghanistan since 2001 after a suicide blast claimed another three soldiers' lives.

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 yesterday.

The Prime Minister 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 yesterday 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 tonight, 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.

News of the 100th fatality revived calls from some quarters for all British troops to be pulled out of the troubled country.

But Armed Forces chiefs and senior MPs spoke of their pride in the sacrifice made by UK service personnel and the value of their mission.

Air Chief Marshal Sir Jock Stirrup, Chief of the Defence Staff, asserted that the British military effort in Afghanistan was starting to achieve real change.

He said: "Every one of those deaths is a tragedy. Nothing can ever compensate for the loss felt by their loved ones and to them all I extend my deepest sympathies.

"I only hope that the terrible hardship that they have been asked to bear can be eased by the certainty that in Afghanistan our forces are engaged in a most worthy and noble endeavour."

Listing examples, he said previously lawless areas of Afghanistan now had governance, more than seven million children were now in school and growing numbers of Afghans had access to healthcare.

The risks they bear and the sacrifices they make should be in our thoughts, not just today but every day Gordon Brown

Sir Jock continued: "Nowhere is the battle for the future of Afghanistan more pressing than in Helmand, the focus of the British effort, where UK forces have magnificently taken the fight to the Taliban and put them on the back foot.

"Make no mistake, the Taliban influence is waning, and through British blood, determination and grit, a window of opportunity has been opened."

But he warned that this progress could "quickly unravel" without a "continuing and energised" international commitment.

Brigadier Mark Carleton-Smith, commander of Task Force Helmand, said the 100th death was an "unwanted milestone" and a "tragic reminder of the human price" British forces are paying.

But he continued: "We need to set it in the perspective of what we are seeking to achieve here.

"We want to leave Afghanistan for the Afghans in a country that is better-governed, more secure, more peaceful, and crucially no longer represents a threat to the international community.

"I think that is a really significant prize."

The use of suicide tactics showed the Taliban "no longer enjoy the popular support of the Afghan people", he said.

Defence Secretary Des Browne expressed his sympathy for the relatives, comrades and friends of the three soldiers killed yesterday.

He said the 9/11 attacks on America demonstrated how all 100 British military personnel killed in Afghanistan had given their lives securing freedom and stability for everyone.

Mr Browne also spoke of the "awe and admiration" for the courage and dedication of British troops felt by all visitors to Afghanistan.

"As a nation we have always been supremely proud of our Armed Forces, and with considerable justification. Quite simply they exemplify the very best qualities of the human spirit," he said.

"Their effect on southern Afghanistan in the last two years has been remarkable.

"They have transformed the heartland of the Taliban from an area of lawless oppression and terrorism to a place of democracy and development."

Conservative leader David Cameron added: "My heart goes out to their families at this time. We owe so much to young servicemen and women who risk their lives to fight on our behalf.

"The death toll having now reached 100 serves as a tragic reminder of how brave and courageous they are."

But the father of a British soldier killed in Afghanistan two years ago predicted that the conflict would turn into "as big a disaster as Iraq".

Captain Jim Philippson, 29, of 7 Parachute Regiment Royal Horse Artillery, died in a firefight with Taliban troops in Helmand Province on June 11, 2006 in which British forces were out-gunned.

An inquest in February heard that his comrades had complained repeatedly about a lack of proper equipment - chiefly standard night vision kits and weaponry - before his death.

His father, Anthony Philippson, from St Albans, Hertfordshire, said yesterday's deaths would not be the last.

He added: "They have never learnt from Vietnam. The trouble with guerrillas is they never wear uniforms - they disappear into the trees then come back and fight when they want to.

"The whole thing is an utter waste of time."

Stop the War Coalition spokesman Chris Nineham described the conflict in Afghanistan as a "war without purpose" to support US President George Bush's foreign policy.

He said: "Afghanistan is becoming a disaster all too reminiscent of Iraq.

"It is quite clear that the Nato forces are bogged down, deeply unpopular, facing more and more resistance and causing mayhem rather than bringing democracy.

"We want the troops to be withdrawn because I do not think there is any possibility of progress in Afghanistan while it is occupied by foreign forces."

Of the 100 British service personnel killed in Afghanistan since operations began in October 2001, 74 died as a result of hostile action.

The remaining 26 died from illness, accidents, non-combat injuries or have not yet been formally assigned a cause of death.

The youngest victim was Private Ben Ford, 18, from Chesterfield, Derbyshire, and the oldest was father-of-five Senior Aircraftsman Gary Thompson, 51, from Nottingham.

The single worst tragedy of the war was on September 2, 2006, when 12 RAF personnel, one Royal Marine and one soldier were killed when their ageing Nimrod MR2 aircraft came down 12 miles west of Kandahar in southern Afghanistan.

Defence Secretary Mr Browne paid tribute to the courage and professionalism of the British troops serving in Afghanistan where, he said, they were making "significant progress".

"Militarily they have put the Taliban on to the back foot and they have created security and freedom that the people of that area have never known before," he told the BBC Radio 4 Today programme.

He said that while the troops were fully committed to the cause in Afghanistan, there was some frustration that what they were achieving was not properly understood back home.

"There is not one member of our services, not one man, not one woman, not one of those tremendously courageous professional people who we send out there who questions the rightness of this cause," he said.

"Their frustration, if anything, is that people back here in the United Kingdom don't understand what they are achieving and that none of the positives of what they are doing, they believe, are fully understood by people back here."