The ministry said two soldiers from the 1st Battalion Welsh Guards and one from the Royal Corps of Signals were killed Sunday during an incident at Checkpoint Kamparack Pul in the Nahr-e Saraj district of Helmand province.
All three soldiers were part of a police advisory team which had visited the checkpoint to conduct a shura—a meeting of village elders. Defense officials said in a statement that a man wearing the uniform of the Afghan National Civil Order Police opened fire as the soldiers were leaving the checkpoint. They received first aid at the scene but died from their injuries.
British Prime Minister David Cameron said the suspected perpetrator had been detained. "We will do everything in our power, with the Afghan National Security Forces, to ensure that justice is done," he told lawmakers Monday in the House of Commons.
The soldiers killed have not yet been named by officials. A total of 422 British troops have now died in Afghanistan since 2001.
The attack brings the total number of so called "Green-on-Blue" killings involving Afghan security officials attacking NATO allies to 26 in 18 incidents so far this year.
The last Britons to die in such an incident were on May 12 when two soldiers were shot and killed by two men wearing Afghan Police uniforms who turned their weapons against them.
One of the individuals who opened fire was killed when coalition forces returned fire. The second individual fled into Lashkar Gah, the capital city in Helmand province.