Five British soldiers were killed when a rogue Afghan policeman turned his gun on them inside a checkpoint.
The UK servicemen, three from the Grenadier Guards and two from the Royal Military Police, died when the officer opened fire at Nad-e-Ali in Helmand Province yesterday.
A manhunt has been launched for the killer, who may have links to the Taliban, according to reports.
The British soldiers were living and working at the checkpoint as part of a team mentoring the Afghan National Police (ANP).
One of the Afghan policemen apparently fired without warning before anyone could respond, then fled the scene.
Sources named the attacker as a man called Gulbuddin and suggested he was connected to the Taliban, the BBC reported.
A UK military spokesman said: “It’s our understanding that one individual Afghan National Policeman, possibly in conjunction with another, went rogue.
“His motives and whereabouts are unknown at this time. Every effort is now being put into hunting down those responsible for this attack.”
Four of the soldiers were killed immediately and the fifth died of his wounds, the Nato-led International Security Assistance Force (Isaf) said.
Another six UK servicemen and two ANP officers were injured in the incident. The troops’ families have been informed.
The British casualties were evacuated to the field hospital at Camp Bastion in Helmand by Medical Emergency Response Teams using Chinook helicopters and a US Black Hawk.
Investigations have been launched by the Royal Military Police, the local chief of the ANP, Isaf and a team from the Afghan Ministry of Interior.
Lieutenant Colonel David Wakefield, spokesman for Task Force Helmand, said: “It is with the deepest sadness I must inform you that five British soldiers were shot and killed yesterday in Nad-e-Ali District.
“Five British soldiers, five of our own, shot down in the course of their duty. They will not be forgotten.”
Prime Minister Gordon Brown paid tribute to the soldiers, describing their deaths as a “terrible loss”.
“My thoughts, condolences and sympathies go to their families, loved ones and colleagues. I know that the whole country too will mourn their loss,” he said.
“They fought to make Afghanistan more secure, but above all to make Britain safer from the terrorism and extremism which continues to threaten us from the border areas of Afghanistan and Pakistan.
“I pay tribute to their courage, skill and determination. They will never be forgotten.
“It is my highest priority to ensure our heroic troops have the best possible support and equipment – and the right strategy, backed by our international partners, and by a new Afghan government ready to play its part in confronting the challenges Afghanistan faces.
“Our troops deserve nothing less. My commitment to them remains unshakeable.”
Defence Secretary Bob Ainsworth offered his condolences to the families of the soldiers and emphasised the need for the country to show “resolve” in supporting the Afghan mission.
“I was extremely saddened to hear of the deaths of these courageous soldiers. My very deepest condolences go out to their families, friends and colleagues as they come to terms with the loss of these outstanding men,” he said.
“It continues to be a difficult year in Afghanistan for our brave people who are operating within the most challenging area of the country.
“We owe it to them to show the resolve that they exhibit every day in building security and stability in Afghanistan and protecting the UK from the threat of terrorism.”
Tory leader David Cameron said: “I was deeply shocked to hear of the deaths of five British soldiers in a single incident in Helmand Province, and the horrific circumstances in which it appears they died.
“I pay tribute, as will the whole country, to their professionalism and their courage, and send my condolences to their families and their friends.”
Peter Galbraith, who left his post as deputy head of the UN mission in Afghanistan amid disagreements over the presidential elections, said the “rushed” bid to train extra Afghan officers for the poll meant such deaths were to be expected.
“It is a terrible tragedy but it is, I won’t quite say inevitable, but it is not surprising,” he told the BBC Radio 4 Today programme.
“The process of police training and recruiting has been very rushed. Normally the police get an eight-week training course. That is actually very short and there isn’t a lot of vetting of police before they are hired.
“And actually, in recent months, they shortened the training programme from eight weeks to five weeks because they wanted to get more police boots on the ground in advance of the elections.
“So there was a real rush to recruit an additional 10,000, particularly in the south, particularly in Kandahar and Helmand provinces.
“So it is not totally surprising that people were recruited who may have had Taliban sympathies or were infiltrated into the police by the Taliban although I don’t know yet whether in this particular episode that is exactly what happened.”
The shootings took the UK death toll in Afghanistan since the conflict began in 2001 to 229, and made 2009 the bloodiest year for the Armed Forces since the Falklands War.