Gordon Brown has pledged to push ahead with the "vital" mission in Afghanistan as the UK death toll passed the 200 mark.
The Prime Minister said news that the grim milestone had been reached, after the 200th and 201st soldiers lost their lives, was "deeply tragic".
But he told reporters: "In these moments of sorrow and sadness, we must never forget why we are in Afghanistan and why people are making the sacrifice that they are making."
The Prime Minister continued: "Three-quarters of the terrorist plots that hit Britain derive from the mountain areas of Pakistan and Afghanistan and it is to make Britain safe and the rest of the world safe that we must make sure we honour our commitment to maintain a stable Afghanistan."
Amid controversy over equipment levels for UK forces - particularly helicopters - Mr Brown also stressed that the Government would provide support.
"I want everybody to know today that every effort that we make is to ensure the best security and the best equipment for our troops," he said.
"That's why we've increased dramatically the resources we are spending in Afghanistan to deal with this new kind of threat, which is the roadside bomb, the electronic devices, the guerrilla warfare being practised by the Taliban.
"We will honour and support those who have been killed or wounded in the field of battle. And we will give those who fight on all the support that they need to succeed in this vital mission."
