The killings were sudden and savage. There was no escape from the raking bursts of machine-gun fire for the soldiers hemmed in by the walls of the police checkpoint.

The British troops at Blue 25 had no chance to defend themselves, and a terrible price was paid. Four men lay dead, and six others were injured, one of them to die later. Their attacker, an Afghan policeman, was also wounded, but managed to escape on a motorcycle under covering fire from his accomplices.

One of the men who died was yesterday named by his family as Sergeant Matthew Telford, of the Grenadier Guards, who was the father of two sons, aged four and nine. He had been in Afghanistan two weeks. Another was named as 18-year-old Guardsman James Major. The others who fell were Warrant Officer Darren Chant, also of the Grenadier Guards and Acting Corporal Steven Boote and Corporal Nicholas Webster-Smith, both of the Royal Military Police.

The deaths bring the total number of British forces killed this year to 95, of which 92 have been in Afghanistan. The killings of three Guardsmen and two members of the Royal Military Police at Nad-e-Ali in Helmand matched the previous biggest loss suffered by UK forces in Afghanistan in a single attack. Even in this particularly savage war the attack has shocked British troops. They now have to work with the spectre of a fifth column — infiltrated amongst their supposed allies against the Taliban.

The attack also comes in the middle of an intense debate about the war. Even as Barack Obama considers sending 40,000 more troops to the country there is rising opposition to further involvement. The victory awarded to Hamid Karzai after an election riddled with vote stuffing has led to questions about why British, American and other Nato troops should fight and die to prop up an Afghan government internationally labelled as corrupt.

The fact that the Nad-e-Ali killings were carried out by an Afghan policeman has raised fundamental questions about the West's exit strategy — which is to train Afghan forces to take over security in their own country.

The massacre was the latest in a series of attacks on Western troops and officials involving the Afghan police. Four weeks ago a policeman in Wardak opened fire on American soldiers on patrol, killing two of them before fleeing. Last year, Afghan police twice attacked US forces, killing two soldiers and wounding three others. Last week men in police uniforms forced their way into a guest house in Kabul and murdered five UN election workers. They had detailed information on the target of their attack, which, say investigators, could only have come from official sources.