Two soldiers serving with the 1st Battalion Welsh Guards have been killed in Afghanistan, the Ministry of Defence has confirmed.
The MoD says they were shot by a man wearing an Afghan police uniform at Check Point Kamparack Pul in Nahr-e-Saraj, Helmand Province.
They received first aid at the scene but died of their injuries.
A third soldier serving with the Royal Corps of Signals has also been killed.
The soldiers’ families have been informed and have asked for a 24 hour period of grace before further details are released.
The Chief of Staff for Task Force Helmand, Major Andy Cox, has paid tribute to the three soldiers killed.
Defence Secretary Philip Hammond says an investigation into the deaths of three soldiers in Afghanistan is underway.
In a statement, he said: "We are determined to get to the bottom of exactly what happened yesterday at Checkpoint Kamparak Pul, and to pursue those who are responsible.”
He added that although yesterday's incident was 'deeply tragic', it would not 'derail the mission or distract us from the task in hand'.
General Sir David Richards, Chief of the Defence Staff, has also paid tribute to the soldiers but echoed Mr Hammond's statement that what happened 'will not stop us from moving forward.'
Prime Minister David Cameron called news of the soldiers' deaths 'appalling' and said he'd do all he could to ensure justice is done.
The deaths bring the total number of members of UK forces to have died since operations in Afghanistan began in October 2001 to 422.