Two British soldiers have been shot dead in Afghanistan by a man wearing a local police uniform, the MoD has said.
The troops, from 3rd Battalion The Yorkshire Regiment, were killed at a checkpoint in the south of Nahr-e Saraj, Helmand province, on Saturday. The men's families have been informed.
The killer had claimed to be injured and fired on the soldiers as they went to help him, the BBC understands.
Spokesman Maj Laurence Roche said the regiment had suffered a "deep loss".
The number of British military personnel killed since operations began in Afghanistan in 2001 is 430.
Maj Roche said: "Everyone serving within Task Force Helmand will want to send our condolences to the soldiers' families and loved ones at this time."
Meanwhile another four soldiers with the Nato-led force in Afghanistan have been killed in a separate attack believed to have been carried out by Afghan police officers, according to Nato. These soldiers were not British.
The killings also followed the death of another British soldier killed in southern Afghanistan after his vehicle hit a roadside bomb a day earlier.
The soldier, from 1st Battalion Grenadier Guards, was travelling in Nahr-e Saraj. He has not yet been named by the Ministry of Defence but his family has been informed.
The Ministry of Defence said the incident was not connected with a Taliban attack later that evening on Camp Bastion, which is the main base in Afghanistan for UK troops as well as many from other nations. Two US marines were killed in that attack.
Prince Harry was at the base at the time of the attack but was said not to have been in any danger.
The MoD has not confirmed whether the man who shot the two British soldiers on Saturday was an Afghan policeman, but it is thought the incident could have been a "green-on-blue" attack.
BBC defence correspondent Jonathan Beale says he understands the Afghan man responsible for the attack had claimed to be wounded, prompting members of the British patrol to go to help him.
As the soldiers approached him he opened fire. He killed two British soldiers before he was himself killed in return fire, our correspondent says.
The term green-on-blue refers to the colour-coding systems used by the US military - Nato forces are "blue" and Afghans are "green".
The number of Nato soldiers killed in this sort of insider attack stands at more than 50 for this year alone, with seven British soldiers known to have been lost in this way in 2012.
Our correspondent said the trend of green-on-blue attack was clearly worrying and was not going to stop, despite increasingly thorough checks being put in place by authorities.
Some 700 members of the Afghan security forces have been ejected in the past month as part of the checks, according to Afghan officials. But ultimately, it is very hard to guarantee against this type of killing, he added.
Earlier in the week, Defence Secretary Philip Hammond met Afghan President Hamid Karzai and said he was "reassured" that Afghan leaders were taking steps to stop attacks by their troops on foreign soldiers.
After the meeting, the defence secretary said improved vetting procedures for Afghan soldiers, an anonymous system for reporting incidents and outlawing the sale of military uniforms and weapons were among measures being taken by the authorities in Kabul to counteract the problem.
"The green-on-blue threat and the tragic incidents we have seen are a very serious concern to me and the issue was at the top of my agenda in meetings with Afghan leaders," he said.
"I recognise that we cannot eliminate the risk entirely, but I was reassured that President Karzai and the rest of the Afghan government and military hierarchy clearly take this issue as seriously as I do and are taking real steps to tackle the threat.
"We are all united in the view that we cannot let these few terrible incidents derail the steady progress that is taking place."