Michael Howard has challenged the prime minister over the possible rapprochement with Ken Livingstone following the London mayor's blistering attack on George W Bush.

The Tory leader gave his full support to Tony Blair's alliance with his US counterpart, but questioned how this was compatible with Livingstone's comments.

Ahead of talks with the US leader, Blair said the relationship between Britain and America was in both nations' interests.

"I am sure everybody who has the best interests of this country at heart recognises that the strength of the alliance between this country and America is important for world peace, world security and for the future of both our countries," he said.

But Howard pressed on - demanding how the two positions were compatible.

"How then can he support as his party's candidate as mayor for London, somebody who said president is 'the greatest threat to life on this planet than we have most probably ever seen'," he said.

Blair claimed that the opposition leader was "trying to make mischief" - but defended the right of those who will protest against the state visit.

"There will be people in my own party and elsewhere who will be opposed to what the president of the United States has done," he said.

"I happen not to be one of them. And as opposed to making mischief... perhaps this would be the time when both of us should stand firm in support of the alliance between this country and America."

Charles Kennedy, however, warned the prime minister that he would have to stand firm to stop the US administration making an early exit from Iraq.

In reply Blair said that both London and Washington wanted to see free and fair elections in Iraq as soon as possible.

"I hope he would recognise that none of that would have been possible unless Saddam Hussein had been deposed," the prime minister told MPs.

"The people bombing the UN, the Red Cross, killing ordinary Iraqis. the people who are killing people in Turkey and Saudi Arabia. They aren't the British. They aren't the Americans.

"It really is about time we realise who our allies are, who our enemies are, stick by one and fight the other."

Kennedy also challenged Blair to "give us some sense of which issue he's hoping to make some progress on" during Thursday's talks with the president.

Blair later told MPs that he would raise the controversial issue of trade tariffs with the president.

"These trade issues come up from time to time between Britain and America," added the prime minister.

Blair said the trading relationship between the UK and the US was a "very good two way deal... but we want it to be a perfect one".