A total of 132 British troops have been killed in Afghanistan since 2001
Four Royal Marines killed in Afghanistan have been named by the Ministry of Defence.
Sergeant John Manuel, Corporal Marc Birch and Marine Damian Davies died in a blast in Helmand province caused by a suspected suicide bomber aged 13.
Earlier on Friday, Lance Corporal Steven Fellows had been killed in an explosion while on routine patrol in a Jackal armoured vehicle.
On a visit to Afghanistan, Gordon Brown said they "will never be forgotten".
The prime minister also expressed his "disgust and horror" at the Taleban's "cowardly" use of a child.
The explosion killed Sgt Manuel and Cpl Birch at the scene, while Marine Davies died later in hospital at Camp Bastion.
Sgt Manuel, 38, from Gateshead, Tyne and Wear, and 26-year-old Cpl Birch, from Kingsthorpe, Northamptonshire, were serving with 45 Commando.
Marine Davies, 27, from Telford, Shropshire, was with the Commando Logistics Regiment.
L/Cpl Fellows, 26, from Sheffield, was also a member of 45 Commando.
The incidents brought the number of British military deaths in Afghanistan since the start of operations in 2001 to 132.
A veteran of campaigns in Iraq, Belize and Kosovo, Sgt Manuel was nearing the end of his career in the Marines, and hoped to join the police force.
Maj Richard Maltby, his company commander, said the men's thoughts were with his partner Rachel.
"A limitless 'ball of fire', Sgt Manuel was at the forefront of company life," he said.
His friend, Sgt Sean McKeown, who knew him for nearly 20 years, said: "He had the courage of a lion and the personality of a hundred men."
Cpl Birch was described as an "outstanding marine"
Cpl Birch had only recently married, and his comrades spoke of his devotion to his wife Charlene.
Marine Joe Hawley said: "He was devoted to his wife Charlene and extremely close to his family. It was clear to all of us that he loved them very much."
Lt Col Jim Morris, his commanding officer, described him as an outstanding marine.
"Courageous, determined, warm-hearted and with an impish sense of humour; he demanded high standards from his men but always motivated them by personal example and commitment," he said.
Marine Davies leaves behind a young son Matthew and wife Joanne, who is expecting their second child.
His squadron commander, Major Marcus Taylor, said he was an experienced and popular member of the unit.
"Marine Davies was always at the forefront of activity and the first to volunteer from his troop no matter how difficult or dangerous the mission," he said.
Colleagues described Damian Davies as a "Marine's Marine"
Comrades Corporal Ben Parks and Marine Lee Douglas said he was a "Bootneck's Bootneck" - a marine's marine.
"No matter what the task he would make sure it was seen through to the end, never afraid of getting his hands dirty and not happy unless it was completed perfectly," they said.
L/Cpl Fellows, known as Jamie, was described as a "wonderful husband" by his wife, Natalie.
"Jamie was my childhood sweetheart, he meant the world to me and I am devastated," she said.
Maj Ross Preston, his company commander, said he considered himself very lucky to have served with L/Cpl Fellows.
"He embodied everything that it is to be a Royal Marines Commando. Tough and courageous under fire, he was an example to his comrades, fearlessly taking the fight to the enemy," he said.
Speaking at the 45 Commandos base in Arbroath, Scotland, Major Spike Kelly paid tribute to the dead men.
"These men were highly professional and thoroughly committed commandos and their loss will be felt extremely keenly.
"They were an enormous credit to their unit, to the Royal Marines and to the Armed Forces as a whole."
Scottish First Minister Alex Salmond said the loss would be keenly felt in Arbroath.
"This is a dreadful tragedy, and our heartfelt condolences go to the families and friends of the men who have lost their lives," he said.
"But it is a close-knit local community which will rally round all those most closely affected."