Failure in Afghanistan would be a “disaster” for Britain, the Government has warned amid renewed questions over whether the military campaign can ever be successful.

Defence Secretary Bob Ainsworth said the country needed to “show some resolution” as opinion polls indicated public support for the mission was waning in the face of continuing heavy losses.

Shadow foreign secretary William Hague admitted the Conservatives were “very worried” about the prospect of having to take over such a difficult situation in Afghanistan if they came to power in an election next year.

The head of the armed forces, Air Chief Marshal Sir Jock Stirrup, acknowledged that progress was “painful, slow and halting” but he insisted that the mission was “do-able”.

It has been another bloody week for British forces, which began with the killing on Tuesday of five soldiers by an Afghan policeman who they had been helping to train.

The latest losses cast a grim pall over the annual Remembrance Sunday commemorations and prompted a renewed round of national soul-searching over whether the mission was worth it.

A ComRes poll for the BBC1 Politics Show found that almost two thirds — 64% — now believe that the war is “unwinnable”, while a similar proportion — 63% — wanted British troops to be withdrawn as soon as possible.

Mr Ainsworth acknowledged that public support for the campaign had been “dented” but, in some of his starkest comments to date, he insisted that Britain could not afford to give up.

“We have to persevere, we have to show some resolution,” he said. “Failure will be a disaster for us.”

While the Conservatives continued to back the campaign, Mr Hague warned that it would be impossible to sustain public support unless there were clear signs of progress on the battlefield.

“It is a very difficult situation. I would be kidding if I said to you that we weren't very worried about it in the Conservative Party,” he said.

“We have to do much better — and much better in terms of actual military success and showing that progress is being made.”

Air Chief Marshal Stirrup, the Chief of the Defence Staff, confirmed that it would be another four or five years before the international coalition could hand over responsibility for security to the Afghans. However, he said more needed to be done to explain to the public what exactly was being achieved by British troops.

“We have got to do much better at describing their progress. It is painful, it is slow, it is halting, but it is in the right direction,” he told BBC1's The Andrew Marr Show.