A suspected Afghan insurgent who died alongside a Royal Marine and a female soldier was not on patrol with them when they were killed on Wednesday, the Ministry of Defence (MoD) has said.
The Royal Marine, from 40 Commando and the soldier, from 3 Medical Regiment, were on patrol in the Nahr-e Saraj district of Helmand province when they were killed. Their families have been informed.
An Afghan man also died in the incident, which was understood to have been claimed by the Taliban as a "green on blue" attack. But the MoD on Thursday said the man - although believed to be a member of the Afghan Uniformed Police - was not wearing uniform at the time and was not on patrol with British troops.
An MoD spokesman said: "On October 24, during a UK foot patrol in the Nahr-e Saraj district of Helmand Province, there was an exchange of gunfire that resulted in the deaths of a Royal Marine from 40 Commando Royal Marines, a female soldier from 3 Medical Regiment and an Afghan man who is believed to be a member of the Afghan Uniformed Police but who was not wearing uniform at the time.
"The UK patrol were not working with any Afghan partners at the time."
The spokesman said an investigation is ongoing into what initiated the exchange of gunfire but the situation "remains unclear".
The soldier is a 25-year-old woman from Comber in Co Down, Northern Ireland, the Belfast Telegraph said, though the MoD was unable to confirm this.
Major Laurence Roche, spokesman for Task Force Helmand, said: "I am extremely sorry to announce the deaths of a Royal Marine from 40 Commando and a soldier from 3 Medical Regiment serving with Task Force Helmand.
"This is dreadful news for all of us serving in Afghanistan. Our sincere condolences go to their families, friends and colleagues at this time of grief."
Their deaths take the total number of UK service members to have died since operations in Afghanistan began in October 2001 to 435.