Three soldiers were killed in a blast and one in enemy fire, the MoD said
Four British personnel have died from injuries sustained in Afghanistan, the Ministry of Defence (MoD) has said.
Three soldiers from 1st Battalion The Rifles were struck by a blast in the Gereshk district of Helmand province and pronounced dead at the scene.
The fourth, a Royal Marine from 45 Commando, was hit by enemy fire near Sangin and died in a UK hospital.
The deaths, in one day, take the number of UK servicemen and women killed in Afghanistan since October 2001 to 149.
It is believed the three soldiers in Gereshk either set off a landmine or a wire-triggered improvised explosive device.
They were on a routine escort operation.
A medical helicopter was scrambled but all three were pronounced dead.
Commander Paula Rowe, spokeswoman for Task Force Helmand, said after that incident it was an incredibly sad day for the whole force.
"We will all feel the loss of these brave soldiers, whose role was to build the capacity of the Afghan National Army," she said.
"But it is their family, friends and loved ones, as well as the men and women who served alongside them, who feel the greatest pain and we offer them our deepest and heartfelt condolences, thoughts and prayers."
The Nato-led International Security Assistance Force (Isaf) said the soldiers were killed by an improvised explosive device.
It was the worst loss of life in a single incident for UK forces since December.
Spokesman Brig Gen Richard Blanchette said: "It is with deep sorrow that we announce the death of these [three] soldiers and offer our condolences to their grieving families.
"Their valiant efforts will not be forgotten, and spurs us to continue our vital mission to bring security to the people of Afghanistan."
The Royal Marine was injured on a reassurance patrol in northern Helmand on Monday, the MoD said.
He was treated on the ground and at Isaf's medical facility at Kandahar Airfield, before being returned to the UK for further specialist treatment.
He died from his wounds in Selly Oak Hospital, Birmingham, on Wednesday.
Commander Rowe said: "The death of this brave Marine is a tragedy and his loss will be felt deeply by his family, friends and all those who served alongside him."
The next of kin of all four men have been informed.
BBC defence correspondent Caroline Wyatt said improvised explosive devices were an increasing cause of death amongst UK soldiers.
"We understand the men may have been travelling in a Wmik, which is an armoured Land Rover," she said.
"But obviously if you have a bomb of a certain size, that can still be triggered despite the kind of counter measures they will have on those vehicles."