Afghan policeman kills three British soldiers
Major Andy Cox said the loss would be felt deeply across the Helmand Task Force.
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UK troops in Afghanistan
Military deaths in Afghanistan
British forces at the sharp end
British servicemen voice fears
A family of Afghanistan's fallen
Three British soldiers have been killed by a policeman in southern Afghanistan, the Ministry of Defence (MoD) has said.
Two served with the 1st Battalion Welsh Guards and one with the Royal Corps of Signals. Next of kin have been told.
The MoD said the soldiers were shot and fatally wounded on Sunday as they left a checkpoint in Helmand province. The gunman was injured and later detained.
More than 20 foreign personnel have been killed in rogue shootings in Afghanistan this year.
The soldiers were part of a Nato-led Isaf force who have been training Afghan counterparts ahead of a handover of security responsibility by 2014.
The shooting happened at Checkpoint Kamparack Pul in Nahr-e-Saraj, where the soldiers were attending a meeting of elders.
They were shot as they were leaving the checkpoint.
Prime Minister David Cameron said he was "deeply saddened by the appalling news".
"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," he said in a statement.
Defence Secretary Philip Hammond said all thoughts were with the families of dead men.
"They gave their lives protecting Britain's national security, helping to make sure that Afghanistan never again becomes a haven for international terrorism," he said.
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Green on blue deaths
So far this year there have been 26 "green-on-blue" deaths - mostly Americans
There were 35 such deaths during 2011
The attacks have claimed the lives of 14 British service personnel since 2009
About 130,000 coalition troops are fighting alongside 350,000 Afghan security personnel against the Taliban-led insurgency
However, he signalled that the killings would not prompt an overall change of strategy.
The defence secretary said: "Every day, tens of thousands of coalition forces, including UK personnel, live and work successfully with their Afghan counterparts to build an Afghan police force and Army which can take the lead for their own security by the end of 2014.
"That process will continue, and though deeply tragic, yesterday's incident and attacks like it will not derail the mission or distract us from the task in hand."
Those sentiments were echoed by General Sir David Richards, Chief of the Defence Staff, who said: "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 and 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."
But Mark Cann, spokesman for the British Forces Foundation charity, said soldiers were worried about how Afghan colleagues, particularly police, were vetted.
"There is deep concern about the people they're working with and, and looking also to hand over authority to. Generally though what I hear is, is a very high level of regard for those they have worked with operationally - more with the military than the police," he said.
It comes after Maj Ian Lawrence, spokesman for Task Force Helmand, announced the deaths in a statement, saying: "Their loss will be felt deeply across the task force. However, that will be nothing compared with the grief experienced by their families at home.
The BBC's Caroline Wyatt said the attacks are a bitter blow for UK forces in Helmand
"Our thoughts and prayers are with them at this extremely difficult time."
BBC defence correspondent Caroline Wyatt says that, despite "green on blue" attacks, background checks are carried out on Afghan troops and police in a bid to ensure that people are not sympathetic to insurgent factions.
In a statement on Sunday, Isaf said: "An individual wearing an Afghan National Civil Order Police uniform turned his weapon against International Security Assistance Force service members in southern Afghanistan today, killing three service members."
The attack appears to be the latest in a string of flashpoints in which members of the Afghan security or police forces have opened fire on international allies.
The latest deaths mean a total of 26 Isaf personnel have been killed so far this year, compared with 35 for the whole of 2011.
A total of 14 British troops have been killed in the past three years in these attacks.
L/Cpl Lee Thomas Davies, 27, from the 1st Battalion Welsh Guards, and Cpl 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 in May.
They had been providing security for a meeting with local officials when two people wearing Afghan police uniforms opened fire.
And a rogue Afghan soldier shot dead Sgt Luke Taylor, 33, of the Royal Marines, and L/Cpl Michael Foley, 25, of the Adjutant General's Corps, at the entrance to the UK headquarters in Lashkar Gar, Helmand Province, in March.
The Taliban claimed responsibility after five British soldiers were killed by a rogue Afghan policeman in November 2009.
The gunman opened fire on the men in a military compound in Nad e-Ali before fleeing.
In February, public opinion against the foreign forces in Afghanistan was inflamed by revelations that US troops burned copies of the Koran at a base in Afghanistan - reportedly by accident.
The shooting of 16 Afghans by a US soldier in March has also created resentment.