The "disaster" of Iraq and Tony Blair's failure to influence US policy will overshadow his time as prime minister, a leading UK think tank has said.
The invasion and the post-war "debacle" have damaged the UK's global influence, said the Chatham House report.
Outgoing director Victor Bulmer-Thomas said Mr Blair's successor would have to build better relations with Europe.
But Foreign Secretary Margaret Beckett told the BBC the "whole thesis of this note is just plain wrong".
She told BBC Radio 4's Today programme: "Nobody said that Tony Blair has so much influence that single-handedly he can solve all the problems of the world."
But, Mrs Beckett added: "The notion that we do not have any influence out there [in the Middle East], or in the European Union, or in the United States is just not true."
She said Mr Blair probably had some "regrets" about what had happened in Iraq but not the "toppling of Saddam Hussein".
The Chatham House report found that, despite military, political and financial sacrifices by the UK, Mr Blair had been unable to influence the Bush administration in "any significant way".
It said there was no evidence British pressure had led to Mr Bush accepting a two-state solution in the Middle East.
"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."
The report, which assessed British foreign policy since 1997, says Mr Blair's successor would also have to rethink Britain's role within the European Union and distance it from the US.
The new prime minister would have to look again at opposition to the euro and the Schengen agreement, which ends controls on international frontiers.
Professor Bulmer-Thomas said Mr Blair's foreign policy during his first term, 1997 to 2001, had been a "qualified success".
The report said the next PM must rethink Britain's European role
He demonstrated his European credentials, while also forging a close working relationship with then US president Bill Clinton.
And Libya's decision to abandon its nuclear weapons programme, along with pushing climate change up the international agenda, counted among Mr Blair's successes, the report added.
But Professor Bulmer-Thomas, who steps down as Chatham House director on New Year's Eve, said: "Despite a number of successes, especially in his first term, Tony Blair's time in office will be overshadowed by the disaster in Iraq."
He said the UK's response to the recent Iraq Study Group findings should be to forge a common European response - or risk international isolation.
In November, comments from a US official that Britain got nothing back from its relationship with the US and that President Bush often ignored Mr Blair were later disowned by the State Department.
Spokesman Tom Casey later said: "The comments, frankly, I think, could be described as ill-informed, and I think from our perspective, just plain wrong."
Visiting Iraq this week, Mr Blair reiterated his determination to stand "four square" behind the Iraqi government in its battle to defeat terrorists.
He told British troops: "If we don't stand up and fight for the people of tolerance and moderation who want to live together, whatever their faith, then the people of hatred and sectarianism will triumph."