Gordon Brown will try to shore up flagging public support for the war in Afghanistan today by telling Britain that it is a campaign “of necessity, not choice”.

The Prime Minister will say that, with al-Qaeda still plotting attacks on Britain from the region, “we cannot, must not and will not walk away”.

Mr Brown will speak amid increasing impatience among British defence chiefs and diplomats at President Obama’s slowness in deciding on a new strategy for Afghanistan. They say that the vacuum it has created has made it open season for the Taleban.

A poll last night showed that opposition to the war has risen markedly in the last fortnight. Only 33 per cent of the British public think that the war can be won, while 57 per cent think that victory is no longer possible, according to the YouGov survey for Channel 4 News, which was carried out after the death of five soldiers on Tuesday and President Karzai’s contested victory.

Two weeks ago 42 per cent thought that the Taleban could be defeated. Last night’s poll found that 35 per cent now think that British troops should be withdrawn immediately, compared with 25 per cent two weeks ago.

While Mr Brown will acknowledge that the summer was hard, he will say that either Britain fights the extremists in Afghanistan and Pakistan or waits for them to carry out more terrorist attacks. He will insist that the campaign is winnable.

Sources have told The Times about their worries over the delay in the announcement of a new American strategy. One senior defence source said: “We owe it to the guys who are being sacrificed in Afghanistan to make it clear that the West is going to stick with the present strategy.”

“It has now been several weeks since General [Stanley] McChrystal [the US commander in Kabul] made his request for 40,000 more troops and President Obama needs to make a clear case for action.”

John Hutton, former Defence Secretary, said: “I believe that we are pursuing the right strategy; there is no better alternative.

“We’ve got to have a decision soon from the US President, as well as a tighter bargain with the Afghan Government for the twin-track strategy we are following.”

Mr Brown spoke to Mr Karzai yesterday and today will set out five tests on which the President must deliver to secure the support of the Afghan people and countries outside. He will have to show in his inauguration speech that he will make progress on security, governance, reintegration and reconciliation, economic development and relations with Afghanistan’s neighbours.

Mr Brown will say of Mr Karzai: “He needs a contract with the Afghan people; a contract against which Afghans, as well as the international community, can judge his success.”

He will tell America and other partners: “We entered together eight years ago. We must persist together. In our different ways we must all contribute; in the end we will succeed or fail together.”

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