Four UK soldiers, one of them a woman, have died in Afghanistan after their vehicle was caught in an explosion.
The woman, believed to a member of the Intelligence Corps, is the first female UK soldier to have died there.
Prime Minister Gordon Brown praised the bravery of the soldiers, who died east of Lashkar Gah in Helmand Province.
The dead soldiers' next-of-kin have been informed. Their deaths take to nine the number of UK soldiers killed in Afghanistan in the last 10 days.
The Ministry of Defence is not expected to name the soldiers until Thursday.
Since 2001, 106 British soldiers died as a result of operations in Afghanistan.
Three of the soldiers were killed in the blast and another was pronounced dead at the British military base at Camp Bastion.
A fifth soldier wounded in the attack, which happened at 1540 local time on Tuesday, is in a stable condition.
Mr Brown told MPs that he offered his "profound condolences" to the families and friends of those who died.
He said the country was proud of the soldiers.
"They were undertaking the most difficult missions in the most dangerous of countries," he said.
"Our troops are a second to none, the best in the world. They are on the noblest of missions, they are fighting for freedom for the Afghans in their own country and for the world in protecting us from terrorism."
Conservative leader David Cameron told the Commons that there was a risk of the American and Nato operations in Afghanistan "pulling in different directions".
He asked Mr Brown: "Can you tell us what progress is being made in turning them into one single unified command?"
The prime minister responded: "Yes there are changes being made in the military command but yes also we must back up the civilian effort which is the key to giving Afghan people a stake in the future."
The latest deaths are the biggest single loss of British lives in Afghanistan since an RAF Nimrod crash in 2006 which killed 14 servicemen.
BBC defence correspondent Caroline Wyatt said the feeling among British military leaders was that the Taleban were being outgunned and that their command-and-control was disintegrating.
The problem is that this has driven the Taleban to measures such as the latest explosion which are very difficult to defend against, she said.
The British army spokesman at Camp Bastion, Lt Col Robin Matthews, said: "While the past week or so has indeed been bruising, and no one would deny that, we must remain fixed on what the strategic imperative is here.
"That is delivering a better life, and progress, to the Afghan people."
He added: "All of us have admiration for the work that both male and female British soldiers are doing here and the courage they are showing, acknowledging the risks that are apparent in this particular part of Afghanistan."
Conservative MP and former infantry officer Patrick Mercer believes more could be done to protect British forces from such attacks.
He said: "If you have off-route mines, roadside bombs, why don't we have the helicopters that are available?
"Helicopters are still vulnerable, but they're not vulnerable to this sort of weapon."
Mr Browne announced this week that British troop numbers in Afghanistan will rise by 230 to a new high of more than 8,000 by next spring.
The new troops will boost protection for UK personnel, improve Afghan security training and aid reconstruction work.
The former head of the Army, Gen Sir Mike Jackson, said on Tuesday that British troops must be prepared to match the "strategic endurance" of the Taleban.
He welcomed the announcement of more troops for Afghanistan, and said it would be wrong to set a time frame on withdrawing forces.
Meanwhile Prince Harry joined 200 service personnel at a memorial service at St Giles' Cathedral in Edinburgh.
They service was to remember the 24 soldiers who died during the deployment of the 52 Infantry Brigade in Helmand from October 2007 until April this year.
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