Five soldiers have died this weekend
Three British soldiers have been killed in Afghanistan, taking the number of UK fatalities to 204.
The Ministry of Defence said the soldiers were from the 2nd Battalion The Royal Regiment of Fusiliers.
They died following an explosion while on patrol near Sangin in Helmand province on Sunday morning. Next of kin have been informed.
Earlier, Gordon Brown admitted it had been "a very difficult summer", but said progress was being made.
Speaking about the most recent deaths, Lieutenant Colonel Nick Richardson, spokesman for Task Force Helmand, said "each and every death is a tragedy".
"Words mean very little in such an extremely sad situation but our thoughts and prayers are with the families and friends of these brave soldiers, " he said.
"We share their pain and mourn the loss of these true British heroes."
Five soldiers have died over the weekend - four from the 2nd Battalion the Royal Regiment of Fusiliers.
The grim milestone of 200 was reached on Saturday when a soldier, from the 2nd Battalion The Royal Welsh, died of his injuries at a hospital in Britain.
Another soldier, from the 2nd Battalion The Royal Regiment of Fusiliers, died after an explosion on Saturday while on foot patrol in Sangin.
Three soldiers were killed by blasts in Helmand on Thursday. They have been named as Lance Bombardier Matthew Hatton of 40 Regiment Royal Artillery, and Captain Mark Hale and Rifleman Daniel Wild of the 2nd Battalion The Rifles.
Mr Brown admitted that more than 30 deaths during July and August, as British troops went on the offensive to shore up security in time for Afghan elections, had made it "one of the most difficult summers yet".
The prime minister said the whole country mourned the loss of its soldiers in Afghanistan.
But he said they were engaged in a "vital" mission to protect Britain from terrorism and maintain a stable Afghanistan.
Defence Secretary Bob Ainsworth told the BBC it was essential to maintain support for the mission.
"The spirit of our armed forces is absolutely indomitable, but everybody in the military knows you have to have the military themselves, you have to have the government, and you have to have the nation as a whole if you're going to succeed.
"That trinity has to be maintained. And I just want to urge people: this is difficult; it isn't going to be a short engagement. It needs not only bravery, but patience as well."
The BBC's defence correspondent Caroline Wyatt in Helmand said flags were still flying at half mast for the two soldiers who died on Saturday but UK troops would not be deterred from their work.