Tony Blair has defended Britain's relationship with the US, after claims he had failed to wield any significant influence over President Bush.
Mr Blair said moving away from the US would have a devastating impact on the UK's international standing.
Speaking in Dubai he said the close relationship was a "strength" and had been a cornerstone of policy for years.
The Chatham House report said his legacy would be the "disaster" of Iraq and his failure to influence the US.
Mr Blair, who is on a tour of the Middle East focusing on the peace process, said he had not read the report by Britain's leading foreign policy think tank.
But he said that one of the greatest myths in politics was that close ties with America harmed Britain's attempts to solve problems in the Middle East.
"People know that, whether it's Israel/Palestine, or it's sorting out the economic problems of the region, or it's on any of the major challenges that face our world today - from climate change through to world trade through to global poverty through to the fight against terrorism - whether in Afghanistan, Iraq or elsewhere - it can't be done without America."
He added: "If you back away from [the relationship with America], and give it up because you get under pressure from parts of the media or public opinion, we'll pay a very heavy price in the future.
"That's my belief as prime minister and, if you really want to know, I think that most people when they're actually in the position, having to take the decisions, will come to the same view".
In his report, outgoing Chatham House director Victor Bulmer-Thomas said the Iraq invasion had been a "terrible mistake" and, along with the post war "debacle", had damaged the UK's global influence.
He said Mr Blair's successor would have to build better relations with Europe, and distance themselves from the US.
The report found that, despite military, political and financial sacrifices by the UK, there was no evidence Mr Blair had been able to influence the Bush administration in "any significant way".
Iraq would overshadow Mr Blair's record, said the report
"Blair has learned the hard way that loyalty in international politics counts for nothing," said Professor Bulmer-Thomas.
"And his successor will not make the same mistake of offering unconditional support for US initiatives in foreign policy at the expense of a more positive relationship with Europe."
Foreign Secretary Margaret Beckett told the BBC the report was "just plain wrong".
But, in a Commons debate, Tory MP Andrew Mackay attacked her comment that the paper was not typical of Chatham House studies and contained "flawed" examples to support its claims.
He said: "Chatham House is a hugely influential, respected institution which is totally non-political, totally unbiased, and to have their report dismissed in some petulant, damning terms by the foreign secretary says more about the foreign secretary than it does about Chatham House."