In a Washington Post and ABC News poll, the latest of a cluster of opinion polls since Colin Powell, the secretary of state, presented his evidence against Iraq at the UN security council, 57% of those questioned backed an invasion of Iraq in the face of UN opposition if "some US allies such as Great Britain, Australia and Italy" supported Washington.

In a sign that Britain's support is more crucial than ever to Mr Bush, US opinion surveys showed that solid support for military action depended on having Washington's closest allies by its side.

Without that "coalition of the willing", American public backing for the invasion dropped to 50%, but that still represented a significant increase from the 37% who backed such action in December. Among those who watched Mr Powell, there was 55% support for going ahead without UN blessing, against 41% support among those who did not see the presentation.

The public say they trust Mr Powell on the Iraq issue more than they trust the president himself. The increase in enthusiasm for action was steepest among those who had seen Mr Powell's performance. A Time magazine survey found that more than seven in 10 Americans think he made a strong case for war. In the Washington Post poll, 63% of those questioned thought the US had made its case for getting rid of Saddam Hussein, compared with only 48% who thought so in January .

The boost will help Mr Bush in his current diplomatic game of bluff with his opponents on the security council, led by France. US and British officials believe that French opposition will ultimately crumble once it is clear that Paris is faced with the choice of joining the US drive to war or accepting a highly damaging rift in the UN and a significant blow to its relevance.

France and its supporters have been counting on the fact that domestic support in both the US and Britain for a war is heavily dependent on Washington and London securing another UN resolution approving military action.

A Newsweek poll also found a 10-point jump in support for military force to 60% over the past few days.

That number jumped to 85% if the US had the "full support of the security council". It dropped to 50% if those questioned were specifically asked their position if the US only had support of "one or two of its major allies". If the US did not even have those allies (of which Britain is universally seen as the closest and most important) support dropped to 37%, with 59% opposed to military action.

Mr Powell's appearance last Wednesday also seems to have helped stem the haemorrhage of domestic approval of Mr Bush's performance as president. In the Washington Post poll, his ratings rose to 64% from 59% in late January.

According to the Washington Post poll, the support for war remains far firmer than the opposition. Two-thirds of those who were in favour of an invasion said there was little chance they would change their minds. But more than half of all the opponents of military action conceded they were wavering and were open to persuasion.

The growing unilateralist temper of US public opinion underlined the threat to Tony Blair's domestic position as British voters accept the case against Saddam Hussein, but not the prospect of early military action to disarm him.

The prime minister is battling against the grain of public opinion - as well as a large section of MPs, activists and trade unions. But after months of voter volatility over Iraq, No 10 can take comfort from the contradictory responses that suggest the public could be swung behind military action, especially if it has UN sanction.

That remains a crucial difference between US public opinion and the wider international community.

The latest poll published in yesterday's Times showed by 57% to 34% that voters are not persuaded that Washington and London have made a convincing case for military action, even though (by 61% to 34%) they accept that Saddam Hussein poses a threat to Britain.

To underline how uncertain voters feel, 86% of those interviewed for the Times in a poll by Populus feel the weapons inspectors need more time, while 47% to 46% also accept that their task has been made "impossible" - the same ratio as trusts Mr Blair to take the right decision even though a bare majority (51%) regard him as "George Bush's poodle".