The two troops from 1st Battalion The Mercian Regiment, part of 1st Battalion The Royal Gurkha Rifles Battle Group, died on Friday afternoon, it said.
They were "killed by small arms fire in Nahr-e Saraj, Helmand. They will be sorely missed and we will remember them," Lieutenant Colonel James Carr-Smith, spokesman for Task Force Helmand, said in a statement.
The deaths bring to 292 the number of British troops killed since operations in Afghanistan began in October 2001.
Britain has around 9,500 troops in Afghanistan, largely battling Taliban militants in Helmand.
Britain's new coalition government has put Afghanistan top of its foreign policy agenda.
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