Prime Minister David Cameron said Wednesday that Britain was paying a "very high price" in Afghanistan after three soldiers were killed in a roadside bomb.
Six British soldiers have now been killed this year in Afghanistan, where British casualties have slowed over the past year.
Britain has not lost so many soldiers in one incident since six were killed by a similar blast in March last year.
They received immediate medical attention at the scene of the blast in Helmand province on Tuesday and were evacuated by air to Britain's main Camp Bastion base but could not be saved, the Ministry of Defence (MoD) said.
The deaths take the total number of British troops killed by enemy action in Afghanistan over the 400 mark, to 401.
"We have paid a very high price for the work we're doing in Afghanistan," Cameron told ITV television.
"It is important work because it's vital that country doesn't again become a haven for terrorists -- terrorists that can threaten us here in the UK."But today our thoughts should be with the families and friends of those that have suffered."
The three soldiers from the Royal Highland Fusiliers infantry battalion died when their vehicle was hit on a routine patrol in the district of Nahr-e Saraj.
"Their deaths come as a great loss to all those serving in Task Force Helmand," spokesman Major Richard Morgan said in a statement.
Their families have been told. Their names will be released in due course.
It is the first time since September 2012 that British troops have been killed by a roadside bomb, which have accounted for many of the British deaths in Afghanistan.
There was no immediate claim of responsibility but Taliban militants frequently use roadside bombs against foreign troops and their Afghan allies.
The deaths brings to 401 the total number of British troops killed as a result of hostile action since operations in Afghanistan began in October 2001.
A total of 444 British troops have lost their lives in the campaign.
The MoD said that security in Helmand, a hotbed of the Taliban insurgency, was improving but that it remained a risky and dangerous environment for British troops.
Afghan police and soldiers are taking over responsibility for security, but there is growing concern over the war-torn country's prospects after 2014 when all foreign combat deployments will end.
General Richard Dannatt, the former head of the British army, was asked Wednesday if it was harder to justify the deaths given the fact British forces will soon withdraw from Afghanistan.
He told BBC radio: "It certainly makes deaths like these ones more painful as we are close to the end.
"Often those who we are fighting increase their efforts towards the end to try and play up the fact that they have driven us out or to increase their hand in bargaining at the negotiating table subsequently."