Three British soldiers have died after a man wearing an Afghan police uniform shot them at a checkpoint in Helmand province, the Ministry of Defence has said.
The soldiers, two of whom were serving with the 1st Battalion Welsh Guards and the other who was serving with the Royal Corps of Signals , received first aid at the scene after yesterday's attack but died of their injuries.
They had been working in an Afghan police advisory team.
Their families have been informed and have asked for a 24 hour period of grace before further details are released.
Prime Minister David Cameron said: "This tragic incident again demonstrates the very real risks that our brave soldiers face every day. We will do everything possible to find out how this happened, and learn any lessons for the future."
Major Andrew Cox, chief of staff for Taskforce Helmand, said: "Their loss will be felt deeply across Task Force Helmand. However, this will be nothing compared to the grief experienced by the soldiers' families. Our thoughts and prayers are with them at this extremely difficult time."
Defence Secretary Philip Hammond said: "I was saddened to hear of this cowardly act by a man wearing an Afghan uniform, which has taken the lives of three brave British soldiers."
General Sir David Richards, the UK's chief of the defence staff, said the resolve of the armed forces would be "unshaken" despite the tragedy.
The suspect is in custody, and an investigation is under way.
The shootings happened on the day Afghan National Security Forces took the security lead in southern Afghanistan.
The deaths bring the total number of members of UK forces to have died since operations in the country began in October 2001 to 422.
The attack appears to be the latest in a string of 'green on blue' attacks in which members of the Afghan security or police forces have opened fire on international allies.
Lance Corporal Lee Thomas Davies, 27, from the 1st Battalion Welsh Guards, and Corporal Brent John McCarthy, 25, of the Royal Air Force, were shot dead at a patrol base in the Lashkar Gar district of Helmand province by members of the Afghan police force in May.
Sergeant Luke Taylor, 33, of the Royal Marines, and Lance Corporal Michael Foley, 25, of the Adjutant General's Corps, were shot dead by a rogue Afghan soldier at the entrance to the UK headquarters in Lashkar Gar in Helmand Province in March.