The Ministry of Defence (MoD) is expected to name four British service personnel who died in one day from incidents in Afghanistan.
A Royal Marine died on Wednesday from wounds sustained on Monday, while three soldiers were killed when a bomb struck their Land Rover.
The marine, from 45 Commando Royal Marines, died in Selly Oak Hospital, Birmingham, as a result of injuries from enemy fire while on a reassurance patrol near Sangin in Northern Helmand, said an MoD spokeswoman.
He received immediate medical attention, both on the ground and at the International Security Assistance Force (Isaf) medical facility at Kandahar Airfield, before being returned to the UK for further specialist treatment.
Task Force Helmand spokeswoman Commander Paula 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. We extend our deepest sympathy, thoughts and prayers to them at this terrible time."
The three soldiers, from 1st Battalion The Rifles, were on a routine escort operation in Gereshk district in Helmand Province when they were hit by the blast.
It is believed their weapon-mounted "WMIK" Land Rover was blown up by either a landmine or a wire-triggered improvised explosive device. An emergency helicopter was scrambled, but all three were pronounced dead. They were part of a team mentoring Afghan forces.
Cmdr Rowe said: "We will all feel the loss of these brave soldiers, whose role was to build the capacity of the Afghan National Army.
"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."
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It was the worst loss of life in a single incident for UK forces since December. The deaths take the number of British servicemen and women killed in Afghanistan since the start of operations in October 2001 to 149.
