The killing of five British soldiers by an Afghan policeman raised fresh doubts yesterday about Britain’s mission in Helmand.

Senior political, diplomatic and military figures warned that public support for the British presence was in danger of collapse without a clear and freshly defined strategy.

The deaths of the soldiers, three from the Grenadier Guards and two from the Royal Military Police, came when a policeman trained by British forces opened fire at Shin Kalay base in southern Afghanistan. Building up the expertise of the Afghan army and police force is key to the British and American forces eventually leaving the country, and that may now be far more difficult to achieve.

The shootings exposed cracks in the military alliance and domestic political unity. Lord Ashdown of Norton-sub-Hamdon, the former Liberal Democrat leader, and Lord Powell of Bayswater, Margaret Thatcher’s former foreign policy adviser, warned of the dangers of ebbing public support.

Lord Ashdown writes in The Times: “There is now a real chance that we will lose this struggle in the bars and front-rooms of Britain, before we lose it in the deserts and mountains of Afghanistan.” Lord Powell said that the public wouldn’t accept a strategy that did not include a cut-off point within three years.

The soul-searching was reflected by the Right Rev Stephen Venner, the Bishop to the Armed Forces. He said: “I would hope that all politicians and church leaders would be asking questions. We are asking our people . . . to be in positions of huge danger. We must always ask the question about whether it is right we should be there.”

At Westminster, Gordon Brown was fighting to maintain Labour unity and Nick Clegg, the Liberal Democrat leader, told The Times that his support for the war was “not unconditional”, hinting that his backing depended on America’s new strategy succeeding.

General Lord Guthrie of Craigiebank, former Chief of the Defence Staff, said the mission had reached a tipping point but blamed President Obama’s delayed decision on whether to send more US troops.

Amid signs of uncertainty about how to proceed, Mr Brown signalled that London and Washington had revived the idea of a senior civilian alliance figure to assist President Karzai as he tries to stamp out corruption. Lord Ashdown, once touted for such a role, said that the West should bypass Mr Karzai and deal with tribal leaders.

In the Commons, Labour MPs voiced sympathy for a call from the former minister Kim Howells for a phased withdrawal of troops.

Grenadier Guards killed were named as Guardsman James Major, 18, Sergeant Matthew Telford, both from Grimsby, and Sergeant Major Darren Chant. The Royal Military policemen were Acting Corporal Steven Boote and Corporal Nicholas Webster-Smith, 24, from Saundersfoot.

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