The serviceman, from 3rd Battalion The Rifles, died today from wounds sustained in a firefight near Sangin in Helmand Province yesterday evening.
This followed the death of Lance Corporal Michael David Pritchard, 22, of the 4th Regiment Royal Military Police, in a separate incident in Sangin on Sunday.
Both deaths may have resulted from friendly fire and they are under investigation by the Royal Military Police, the MoD said.
The soldiers’ families have been informed.
Lieutenant Colonel David Wakefield, spokesman for Task Force Helmand, paid tribute to the latest member of UK forces to die in Afghanistan.
He said: "It is with deep sadness I must inform you that a British soldier from 3rd Battalion The Rifles passed away early this afternoon as a result of wounds sustained during a fire-fight last night near Sangin, Helmand Province.
"His courage and his sacrifice will not be forgotten."
The MoD said it would not release any more information until the conclusion of inquests into the shootings.
The British death toll since the start of operations in Afghanistan in 2001 now stands at 242, including 105 deaths this year.
Tributes were also paid to L/Cpl Pritchard, who was born in Maidstone, Kent, and lived in Eastbourne, East Sussex.
His family said in a statement: "With great sadness we say goodbye to our beloved son, a lover of life who has lived life to the full and has brought great joy to all those who are lucky enough to know him.
"A light that shines brightly, our precious son, brother, grandson, boyfriend and special friend to all, we are very proud of you in all that you have done and achieved and you will always be in our hearts now and ever more.
"God bless our darling boy, from all of your family and friends."
L/Cpl Pritchard joined the Royal Military Police in 2007 and had been in Afghanistan since October.
Lieutenant Colonel Debbie Poneskis, commanding officer, 4th Regiment, Royal Military Police, described him as a "cheeky chappy".
She said: "Lance Corporal Pritchard had only been in the Royal Military Police since July 2007 but he very quickly made a huge impact on all of us.
"It is tragic that we have been robbed of such a promising junior non-commissioned officer and one who was everything you would want in a military policeman.
"He was a professional and robust soldier and one who was both physically and morally courageous.
"He was absolutely committed to providing policing advice where it mattered most, alongside his infantry colleagues on patrol and as part of the team.
"A cheeky chap, whose laughter was infectious and whose sincerity and generous spirit touched the lives of many, Lance Corporal Pritchard made us smile every day and we will miss him very much.
"He was never afraid to speak his mind, even if that sometimes got him into trouble, but he was one of those soldiers you could never really be cross with for long; he had the broadest smile and the most wonderful personality."
Major Phil Hacker, officer commanding 160 Provost Company, said: "Lance Corporal Pritchard was a gregarious, outgoing and hardworking soldier.
"His enthusiasm was matched only by his superb sense of humour. He truly was one of the central characters of the company.
"Utterly professional and wholly dependable, this much-loved soldier will be missed by us all."
Defence Secretary Bob Ainsworth added: "I was deeply saddened to hear of the death of Lance Corporal Michael Pritchard.
"His colleagues describe him as a professional and courageous soldier with a superb sense of humour that made him popular with all in his regiment.
"He was hard-working and dependable and was showing great promise as a junior non-commissioned officer.
"The loss of Lance Corporal Pritchard is keenly felt by his loved ones and those who worked with him and I extend my condolences to his family, comrades and friends."
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