Labour is preparing a break with its recent past by dropping its unquestioning support of US foreign policy under Tony Blair and Gordon Brown.
The party is conducting a review expected to conclude that it made mistakes on Iraq, human rights abuses and some areas of Afghan policy.
In future, the party will suggest, it must be clearer in public where it disagrees with American administrations as well as areas where it agrees.
In an interview with The Independent ahead of her first major foreign policy speech today, Yvette Cooper, the shadow Foreign Secretary, said America was by far the UK's most important foreign ally, but Britain should be prepared to speak out in certain circumstances.
"It is a very important relationship and it needs to be a strong relationship. But there will be a different kind of relationship with different governments," she said. "There will be areas that we strongly agree on, and there will be areas that we strongly disagree on."
Ms Cooper said that under Mr Blair and Mr Brown, ministers did raise concerns with the Bush administration both over human rights and aspects of the war in Iraq, but perhaps should have done so publicly. "If you look at some of the things that George Bush said about water-boarding it's just shocking – really troubling. Guantanamo Bay was a huge problem and area of disagreement. Wherever we did not make that sufficiently clear I think that was the wrong thing to do. We should have said very clearly and strongly that that was the wrong approach."
In her speech to Chatham House today, Ms Cooper will criticise Tory foreign policy on Europe, economic growth strategy and some aspects of human rights. But it is her views about Labour's own policy review which will be of most interest to foreign observers.
Ms Cooper said: "My initial thoughts are we got it wrong on Iraq. I thought there were weapons of mass destruction and there weren't, and that was wrong. We have to recognise that and face up to that.
"But equally I think the government was right on things like Sierra Leone and Kosovo. We were right to work so strongly with other countries throughout the financial crisis. And we were right to increase the support for international aid.
On Afghanistan, she said: "The big question is how far we could have had local political arrangements built into the constitutional discussions and how far they need to be now."