Gordon Brown has pledged to press ahead with the "vital" mission in Afghanistan as the UK death toll hit 201.
The Prime Minister said news that the 200 milestone had been reached, after a soldier died from injuries sustained in a roadside bomb blast, was "deeply tragic".
But he insisted: "My commitment is clear: we must and will make Britain safer by making Afghanistan more stable."
The 200th soldier, from The 2nd Battalion The Royal Welsh, died on Saturday at the Royal College of Defence Medicine, in Selly Oak, West Midlands. He had been on vehicle patrol in Helmand province when the explosion happened on Thursday morning.
Within hours, a further British soldier, from The 2nd Battalion The Royal Regiment of Fusiliers, was said to have died from injuries suffered in a separate explosion.
Mr Brown said in a statement that these deaths were "deeply tragic news". He went on: "Every death leaves a hole in a family's life that will never be filled. We are hugely indebted to those who have made the ultimate sacrifice.
"British troops are fighting bravely there to protect us. The best way to honour the memory of those who have died is to see that commitment through. Three quarters of terrorist plots against Britain come from the border areas of Afghanistan and Pakistan. The work of our troops is vital in preventing al-Qaida once again using Afghanistan as a base for terrorist attacks against Britain and other countries."
Earlier, three British soldiers were said to have died while providing security for a meeting of tribal elders in the run-up to Afghanistan's crucial elections. Two of the servicemen killed by a blast had died trying to save a colleague injured in an earlier explosion.
Lance Bombardier Matthew Hatton, 23, of 40th Regiment Royal Artillery, was wounded along with two colleagues in the initial attack in Sangin on Thursday.
Captain Mark Hale and Rifleman Daniel Wild, both from 2nd Battalion The Rifles, were carrying him to a helicopter landing zone when there was a second explosion, killing L/Bdr Hatton and Rfn Wild. Capt Hale died of his injuries later in hospital.
