A Royal Marine and a female soldier have been killed after a gunfight while on foot patrol in Afghanistan, the Ministry of Defence (MoD) has said.
The families of the Marine, from 40 Commando, and the soldier, from 3 Medical Regiment, have been told.
An Afghan man believed to be a policeman, who was not wearing uniform at the time, also died in the incident in the Nahr-e Saraj area, the MoD said.
Two other British female personnel have died in Afghanistan since 2001.
BBC defence correspondent Caroline Wyatt, in Kabul, said earlier that an Afghan source reported the Britons were killed by an Afghan ally but other locals disputed this.
If the source was correct the deaths would be among a number of "green on blue" attacks on coalition troops this year.
The MoD said the UK patrol was "not working with any Afghan partners at the time".
"At this stage we do not know what initiated the exchange of gunfire and an investigation is ongoing," a spokesperson said.
On Wednesday morning, Maj Martin Creighton, spokesman in Kabul for Nato's International Security Assistance Force (Isaf), outlined what was known about Wednesday's incident.
He told the BBC: "Initial reports, and current operational reports, indicate that this was an insurgent attack in which the British soldier and Royal Marine were killed, but we are aware that there are other interpretations of what happened and Isaf officials are currently working to determine what exactly did happen."
Maj 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."
It is understood the female soldier serving with 3 Medical regiment is from County Down, Northern Ireland.
DUP assembly member Michelle McIlveen tweeted she was "saddened to hear of the death of the brave young female soldier".
A total of 435 British military personnel have been killed in Afghanistan since operations started in 2001.
The first British women to die in action was Cpl Sarah Bryant, 26, from Cumbria, in June 2008. She was one of four soldiers to be killed when her Snatch Land Rover was caught in an explosion in the Lashkar Gah area.
In April last year, Capt Lisa Jade Head, from 11 Explosive Ordnance Disposal Regiment, Royal Logistic Corps died in a UK hospital from injuries suffered in an explosion while clearing roadside bombs in the Nahr-e Saraj area. The 29-year-old was from Huddersfield.