David Cameron has offered his thoughts to the families and friends of six British soldiers "missing, believed killed" in Afghanistan.
A Warrior armoured fighting vehicle was hit by an explosion on 6 March.
Five soldiers from the 3rd Battalion the Yorkshire Regiment and one from the 1st Battalion the Duke of Lancaster's Regiment had been on patrol on Tuesday.
At prime minister's questions on 7 March 2012, the prime minister said it would be the largest loss of life in a single incident in Afghanistan since 2006, taking the overall number of casualties to more than 400.
"Every death and every injury reminds us of the human cost paid by our armed forces to keep our country safe," he told the Commons.
Labour leader Ed Miliband echoed Mr Cameron's comments about the "terrible news" of the six soldiers.
He said it was a reminder of the ongoing commitment and sacrifice British service personnel make.
"By putting themselves in harms way for our benefit they demonstrate the utmost service and courage. We owe them and all those who have lost their lives in Afghanistan an immense debt of gratitude," he said.
Mr Miliband added stressed the importance of restating the reason for the mission in Afghanistan. Mr Cameron thanked him and said he was "absolutely right".
The prime minister added that progress was being made in Afghanistan on various targets.
But the greatest difference would be for a stronger political settlement, he said, which would help ensure there is "real peace and stability" in the country in the future.
The mood in the chamber was sombre as MPs listened quietly to the two leaders.