A man who was dressed as a local Afghan policeman and killed two British soldiers pretended to be injured so they would help him, defence sources have said.
The Ministry of Defence (MoD) said an investigation was under way into the circumstances of the soldiers' deaths at a checkpoint in the south of Nahr-e Saraj and could not comment.
But sources said the killer lured the men from 3rd Battalion The Yorkshire Regiment to their deaths after faking an injury.
The deaths follow that of Lance Corporal Duane Groom, from 1st Battalion Grenadier Guards, who died on Friday after his vehicle hit a roadside bomb.
An MoD spokeswoman said: "It is with sadness that the Ministry of Defence must announce the death of two soldiers from 3rd Battalion The Yorkshire Regiment on Saturday September 15 2012. The soldiers were shot and killed by a man wearing the uniform of the Afghan local police at a checkpoint in the south of Nahr-e Saraj district, Helmand province."
The soldiers' next of kin have been informed, the MoD added.
Major Laurence Roche, of Task Force Helmand, said on Saturday night: "It is with deep sadness that I must report the death of two soldiers from 3rd Battalion The Yorkshire Regiment who were shot and killed by a man wearing the uniform of the Afghan local police at a checkpoint in the south of Nahr-e Saraj.
"The Yorkshire Regiment has suffered a deep loss today and everyone serving within Task Force Helmand will want to send our condolences to the soldiers' families and loved ones at this time."
The soldier who died on Friday was killed when his vehicle hit an improvised explosive device. The MoD said the deaths were unrelated to the attack at Camp Bastion in which two US Marines were killed.
The deaths bring the number of members of UK forces to have died since operations in Afghanistan began in October 2001 to 430.