Five British soldiers have been shot dead in Helmand Province, in an attack the UK military says was carried out by a "rogue" Afghan policeman.
Three of the soldiers were from the Grenadier Guards and two were from the Royal Military Police. Several others were injured.
The officer opened fire at a police checkpoint before fleeing.
It is the highest number of UK soldiers killed in a single incident of "combat" since the US-led invasion of 2001.
An investigation into the attack is under way. The soldiers' next of kin have been informed of the deaths.
Manhunt
A UK military spokesman said: "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."
BBC Kabul correspondent Ian Pannell said sources indicated the attacker was a police officer called Gulbuddin who had fled the scene after the shooting.
It appears he could have been involved in a dispute with his commander, but tribal sources have pointed to a link with the Taliban.
Lt Col Wakefield, spokesman for Task Force Helmand, said the men who were killed had been mentoring and living with a number of Afghan police officers.
He said they had worked and lived in a compound at a national police checkpoint for the past two weeks.
Lt Col Wakefield said: "It is with the deepest sadness I must inform you that five British soldiers were shot and killed yesterday in Nad 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 said the latest deaths were a "terrible loss".
He said: "My thoughts, condolences and sympathies go to their families, loved ones and colleagues. I know that the whole country too will mourn their loss.
"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."
Worst incident
The British Military Police have launched an investigation. The local chief of the Afghan National Police (ANP) and the Afghan national director of security have also begun investigating at the scene.
The Grenadier Guards have been advising the ANP and the Afghan National Army in training, tactics and patrol methods.
The deaths take the number of UK troops killed in Afghanistan since 2001 to 229.
This is the worst single incident in Helmand since 10 July, when five soldiers from 2 Rifles were killed by improvised explosive devices near the town of Sangin.
It brings the number of British casualties this year to 92 - the highest in any year since the Falklands War in 1982.
