'New style, but no policy shift'

Mr Straw was speaking as Mr Brown prepared to launch his campaign to succeed Tony Blair as prime minister later with a speech to activists in London and the unveiling of his manifesto.

The Chancellor will use the speech to stress the moral principles at the basis of his political beliefs and to focus on the "new insecurities" of climate change and terrorism which worry voters today, said Mr Straw.

His comments came as American secretary of state Condoleezza Rice said she expected the US to work "very, very well" with Mr Brown as prime minister.

Ms Rice told BBC Radio 4's Today programme: "I think the relationship will be very close, in part because Britain and the United States are such key allies.

"The bonds with Prime Minister Blair have been forged through some of the most difficult times, through the time of 9/11, through the time of the attacks on London, through Afghanistan and Iraq and Northern Ireland and those are bonds of friendship that come from having been through some of the toughest circumstances.

"But Britain and America will always be friends and I know that we will work very, very well with Gordon Brown when he becomes prime minister."

There has been much speculation that Mr Brown will form a more distant relationship than Mr Blair with US president George Bush, particularly over Iraq. Some Westminster observers believe he will move quickly to withdraw the remaining UK troops from the country.

But Mr Straw told Today: "Gordon Brown's affection and understanding of America goes back decades and he takes the same view that Tony Blair does, which is that Britain's security fundamentally depends on the alliance with the US and the union with Europe."

"You are not going to get a dramatic shift here, but of course you will get a different style of leadership, because Gordon Brown is a different person from Tony Blair," he added.