Prime Minister David Cameron has paid tribute to three British soldiers shot dead by a gunman wearing an Afghan police uniform.
The men, members of an Afghan Police Advisory Team, were leaving a shura - or meeting of elders - at Checkpoint Kamparack Pul in Nahr-e-Saraj, Helmand Province, when they were shot yesterday.
They received first aid at the scene but died of their injuries. Two of the men were serving with the 1st Battalion Welsh Guards and the third was serving with the Royal Corps of Signals. Their next of kin have been informed.
It is the latest in a growing number of so-called "green on blue" attacks in which members of the Afghan security or police forces have opened fire on international allies.
The gunman, who was wearing an Afghan National Civil Order Police uniform, suffered gunshot injuries and was taken into custody.
Mr Cameron said: "I am deeply saddened by the appalling news that three British soldiers have been killed in Afghanistan. My heart goes out to their families.
"These brave soldiers were demonstrating great courage to help prevent Afghanistan once again become a haven for international terrorists and therefore to help keep us safe here in the UK.
"The suspected perpetrator is in custody and we will do everything in our power, with the Afghan National Security Forces, to ensure that justice is done.
"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."
An investigation into the incident has been launched. Four other British soldiers have died in similar circumstances so far this year.
The Ministry of Defence said there had been 16 "green on blue" UK fatalities since 2008, including the seven killed this year. In 2011, there was only one such death.
In May, 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. The servicemen had been providing security for a meeting with local officials when two people wearing Afghan police uniforms opened fire.
Two months earlier, 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 November 2009, five British soldiers were killed by a rogue Afghan policeman. The gunman opened fire on the men in a military compound in Nad e-Ali before fleeing. The Taliban later claimed responsibility for that attack.
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.
"All of our thoughts are with the families of these men who have died in such tragic circumstances. They gave their lives protecting Britain's national security, helping to make sure that Afghanistan never again becomes a haven for international terrorism."
General Sir David Richards, Chief of the Defence Staff, said: "I was extremely saddened to hear of the deaths of these three British soldiers. My thoughts are with their families who will be devastated by their loss and the friends they fought alongside.
"I was in Afghanistan with British troops last week where these deaths will be felt keenly. At the same time, I know that the resolve of those on operations will be unshaken.
"Generally, there remains a high level of trust between the Afghan forces and their British counterparts with whom they work and live every day.
"Every time I visit Afghanistan I am struck by the progress we are making alongside the Afghans in building a country increasingly able to stand on its own two feet. Attacks like this will not stop us from moving forward."
Nato secretary-general Anders Fogh Rasmussen said: "The Taliban has clearly played out a strategy to undermine the confidence in the Afghan security forces but let me also stress that they cannot derail our strategy.
"Our strategy is to gradually hand over full responsibility for security to the Afghans and that process will continue and be completed by the end of 2014.
"This is the reason why it's an essential part of our mission in Afghanistan to try to educate Afghan security forces to build up their capability to take full responsibility and these training efforts will continue."
The deaths bring the total number of members of UK forces to have died since operations in Afghanistan began in October 2001 to 422.
Yesterday's shootings happened on the day Afghan National Security Forces took the security lead in southern Afghanistan. In a ceremony at Camp Hero in Kandahar Province, the International Security Assistance Force handed over responsibility to the body, which includes the Afghan National Army, Afghan Uniformed Police, Afghan Border Police and Afghan National Civil Order Police.
The Regional Command (South) region, which includes Kandahar, Zabul, Uruzgan and Daykundi provinces, is the first area in Afghanistan where such a transition has taken place.