PRIME Minister David Cameron has paid tribute to three British soldiers who were shot dead by a gunman wearing an Afghan police uniform.

The soldiers, 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 on Sunday.

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 Gah 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 Gah 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, echoed Mr Hammond's sentiments, adding: "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."

The number of UK armed forces members to have died since operations in Afghanistan began in October 2001 is now 422.