The soldier was killed as a result of enemy fire while on a patrol in the area of Kakaran, north east of Lashkar Gah.
Despite receiving immediate medical attention, he died from his injuries at the scene, the MoD said.
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An ejector seat manufacturer is to be prosecuted over the death of Red Arrows pilot Sean Cunningham at RAF Scampton in November 2011.
Police said the "active shooter" had since been shot by officers and they were not seeking any other suspects.
Rain will continue spreading in from the southwest, remaining heavy, especially in western areas.