Britain lost its 100th soldier in Afghanistan since the 2001 invasion after a suicide attack in the south of the country killed three soldiers Sunday, the defence ministry said.
The head of Britain's armed forces and the defence secretary both insisted, however, that progress was being made in Afghanistan, and that the soldiers' deaths had not been in vain.
Air Chief Marshal Sir Jock Stirrup, the chief of the defence staff, said he hoped that the families of the 100 soldiers who had lost their lives would find comfort in the fact that "our forces are engaged in a most worthy and noble endeavour".
"Right across the country, the international effort is beginning to effect real change," he said.
The influence of the Islamist Taliban militia was "waning", he added.
"These deaths, though hard to bear, remind us all of the extraordinary sacrifices they and their families make on our behalf -- and of the price of failure if we falter in Afghanistan," Stirrup said.
"We continue to owe them a great debt of gratitude."
Defence Secretary Des Browne, meanwhile, described the progress being made in southern Afghanistan as "remarkable", saying: "They have transformed the heartland of the Taliban from an area of lawless oppression and terrorism to a place of democracy and development."
"Quite simply they exemplify the very best qualities of the human spirit ... We must never forget that this extraordinary achievement, which makes us all safer from the scourge of terrorism, has come at a very significant cost to our brave servicemen and women, their families and friends."
The trio who were killed were on a routine foot patrol about one kilometre (0.6 miles) west of their base in the restive Helmand province, when they suffered a "suicide explosive device".
Their deaths raised the overall number of fatalities among British troops since the 2001 US-led invasion to 100.
A fourth soldier was also wounded in the attack, which occurred at about 11:00 am (0630 GMT) on Sunday.
"Four soldiers were injured in the attack and were evacuated to the medical facility at Camp Bastion," said a defence ministry statement.
"Sadly one soldier was pronounced dead on arrival and despite the best efforts of the medical team, two of the soldiers died as a result of their wounds.
"A fourth soldier is currently receiving treatment for his wounds and is expected to make a good recovery."
Their next of kin have all been informed, the ministry confirmed. They had requested a 24-hour grace period before further details of their identities were released.
All the soldiers, part of the NATO-led International Security Assistance Force, were part of the 2nd Battalion the Parachute Regiment.
Britain has about 7,800 soldiers stationed in Afghanistan, most of whom are in Helmand, where the Taliban have been waging a bloody insurgency since being ousted from power after the 2001 invasion.