A Royal Marine has died after his unit came under fire in Afghanistan, the Ministry of Defence said.
The UK is to increase the number of troops over the summer
The marine, from 42 Commando, was involved in a clearance operation in the Kajaki area of northern Helmand.
The death takes the number of UK troops killed in combat operations in Afghanistan since 2001 to 51.
An MoD spokesman said the marine was not involved in a major Nato assault against Taleban and drug traffickers taking place in the same province.
Brigadier Jerry Thomas, Commander of the UK Task Force, said: "Our marine was killed during a pre-planned operation.
"Whilst the loss of a valued colleague is a tragic blow, our Marines continue to display their professionalism, determination and dedication in carrying out their duties in our area of operations in an exemplary fashion."
The MoD said the marine's next-of-kin have been informed.
The last British deaths in Afghanistan came on Saturday when two servicemen from 29 Commando Regiment Royal Artillery, were killed in a rocket attack in the Sangin area of Helmand.
In February, the government said the UK would deploy 1,400 more troops to the country over the summer, bringing total numbers up to 7,700.
The Nato operation in northern Helmand, involving British and local troops, backed up by US, Dutch and Canadian personnel, was reportedly launched at the request of the Afghan government.
Part of its focus is the area around the Kajaki dam, where the Taleban have been engaging British forces.
The military wants to secure strategically important hydro-electric plant that the so that it can be repaired.