Relaxing after their patrol, with their armour and helmets off, the troops were drinking tea only a few metres from their killer, a man they had been risking their lives to train.

Five British soldiers were killed inside their base in Shin Kalay, southern Afghanistan, on Tuesday when the Afghan policeman sprayed them with machinegun fire. Sergeant Matthew Telford and his men were standing together when the 26-year-old man, named locally as Gulbuddin, opened fire from a rooftop.

His bullets struck at the heart of Britain’s presence in Afghanistan — to train and equip Afghan security forces to take over from the foreign troops.

The men had finished a patrol with Afghan police officers through Nad-e Ali, one of the most dangerous districts in Helmand and home to some of the Taleban’s most infamous commanders. It was about 2pm, the hottest part of the day. Many of the soldiers had just removed their Kevlar helmets and heavy Osprey body armour, officials said, believing that they were safe once back inside the compound, Checkpoint Blue 25.

“They were just stood around having a brew,” said a British official in Helmand. “They get back, relax and debrief. It’s the same after any patrol.”

The soldiers were discussing what they had seen, perhaps telling the Afghan commanders what had gone well and what could have been done better. It is not clear what Gulbuddin’s role was — whether he had been on the patrol or guarding the base.

Apart from the five fatalities at least seven other soldiers were injured, five seriously. The lives of two were in danger last night. Two Afghan policemen were wounded. Three of the dead were from the Grenadier Guards; two were from the Royal Military Police.

A source with close ties to the British military said that the shooting may have been provoked by an argument. He said that a senior non-commissioned officer had been “dressing down” the Afghan police moments before Gulbuddin opened fire.

The killings made 2009 the bloodiest year for British forces since the Falklands conflict in 1982.

The checkpoint commander, called Manan, who was also wounded, said of the attacker: “He became crazy and shot the British. Perhaps he had a Taleban contact and the Taleban told him to do it.”

Witnesses said that the policeman opened fire from the roof of the compound, catching the British troops off guard in the courtyard below. A tribal elder said that Gulbuddin had used a Russian-made PKM machinegun.

Abdul Ahad Helmandwal, the head of the Nad-e Ali district council, said: “He first fired on the commander of the police and his deputy, then on the British soldiers. He escaped on a motorbike.”

British and Afghan forces began a manhunt almost immediately, while medics flew the wounded in three helicopters, RAF Chinooks and an American Black Hawk, to a military field hospital at Camp Bastion.

Gulbuddin had been working for the police since 2007, according to local elders, and graduated from the police academy a year ago. He was originally from Musa Qala, in northern Helmand, and had been stationed in Nad-e Ali for more than 18 months.

Last week he moved to Shin Kalay, where the British troops were based.Fourteen Grenadier Guards and two specialist police trainers from the Royal Military Police had been living alongside about 15 Afghan policemen in a former clinic for the past two weeks. Sergeant Telford had been in Afghanistan only since September 20.

Officials in Kabul said there was nothing to suggest there had been an argument before the shooting started.

A spokesman for the US-led training mission said that all police in the district had undergone an intensive course, known as “focused district development”, within the past year.

Locals said that Gulbuddin had been working at a different checkpoint in Nad-e Ali but eight days ago he abandoned his position after a dispute with his commander, named as Issaqzai, and approached Manan. “Manan said he didn’t have space for any more police but he agreed to go and speak to Issaqzai on Gulbuddin’s behalf,” the tribal elder said. “The British soldiers knew Manan and they trust him. But yesterday, at about 2pm, after prayers, they had just come back from an operation and Gulbuddin shot the British with a PKM.

“Manan climbed up on to the roof to see what was going on and he was shot in the leg. Then Manan’s deputy Khairullah shot Gulbuddin. He was wounded in the leg but he escaped.

“Go a hundred metres outside the checkpoint and it’s Taleban control.”

One British military official told The Times that it was possible that Gulbuddin had had an accomplice who may also have fired into the compound.

Sources close to the investigation at the British headquarters said that Gulbuddin may have been high on heroin at the time of the attack. More worryingly for the future of the partnership between Western and Afghan forces is the possibility that the killer had links to the Taleban. The insurgents claimed responsibility for the attack.

If Gulbuddin was indeed a Taleban recruit who got through the vetting process, the whole partnership and mentoring strategy — the key element of the campaign in Helmand — could be undermined, analysts warned. The incident happened a month after the killing of two US soldiers by a policeman in Wardak province, in the east. A third US soldier was wounded by an Afghan policeman in September.

This year 94 personnel have been killed on active service; the death toll for British forces in the Afghan campaign stands at 229, 198 of them killed in action.

Gordon Brown said: “The death of five brave soldiers in a single incident is 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.”

General Stanley McChrystal, the commander of Nato forces in Afghanistan, said that the incident would be “fully and transparently investigated. We will not let this event deter our resolve to building a partnership with the Afghan national security forces to provide for Afghanistan’s future.”

Times Recommends

• Found: the clue to van Gogh’s ear
• Freud saved by Nazi admirer
• Abramovich woos stars with £3m bash