Ex-US President Jimmy Carter has said he is "disappointed" by the apparently subservient attitude of the British government towards the White House.
Mr Carter, the 39th US president, is an opponent of the war on Iraq
Mr Carter said Tony Blair was a good man, but could have used his influence with President Bush more wisely.
The 81-year-old said there had once been "a very strong voice from London in the shaping of a common policy".
His comments, to the BBC's Newsnight, come with Mr Blair under pressure over the timing of his exit from power.
Mr Blair's close relationship with the White House has caused him difficulties throughout his years at Number 10.
Earlier this week, the prime minister hit out at the "anti-American feeling" of some European politicians, describing it as "madness".
"The reality is that none of the problems that press in on us can be resolved or even contemplated without them [America]," he wrote in a pamphlet published by the Foreign Policy Centre.
But Mr Carter told Newsnight: "I have been really disappointed in the apparent subservience of the British government's policies related to many of the serious mistakes that have been originated in Washington."
Mr Carter, an opponent of the US-led war in Iraq, added: "No matter what kind of radical or ill-advised policy was proposed from the White House, it seems to me that almost automatically the government of Great Britain would adopt the same policy without exerting its influence.
This was the case "in the Middle East peace process, in the case of the Lebanese/Israeli war in the recent past and certainly in the ill-advised abandonment of the war against terrorism to substitute the war in Iraq", he said.
Asked if he thought Britain was exerting its influence behind the scenes, Jimmy Carter replied he had seen no evidence of that.
"I haven't seen the corrective effect of British disagreement with what the White House has proposed. It may be there, it hasn't been evident to the public," he said.
A Downing Street spokesman said Number 10 had nothing to say about Mr Carter's comments, adding that Mr Blair had made clear why he thought the transatlantic relationship was important.
In 1976 Mr Carter unseated the incumbent Gerald Ford to become the 39th US president, serving until 1981.
He won the Nobel Peace Prize in 2002, for what presenters cited as decades of work seeking peaceful solutions and promoting social and economic justice.