Tony Blair effectively subcontracted the decision to invade Iraq to former United States president George Bush, according to a former senior diplomat.
Sir Christopher Meyer, Britain's ambassador to Washington between 1997 and 2003, also claims the former prime minister's "black and white" view of the world fuelled mistakes before and during the invasion.
Writing in the Sunday Telegraph 10 years after the invasion, he described Mr Blair as "more evangelical than the American Christian Right".
"He (Mr Blair) has always argued that, by keeping close to Mr Bush, he was able to influence him," Sir Christopher wrote. "But for Mr Blair the 'special relationship' became an end in itself, leading him to tell Mr Bush that, whatever he decided, he would have Mr Blair's support.
"This was tantamount to subcontracting to Mr Bush the decision to invade Iraq. It was not for nothing that Mr Bush once congratulated Mr Blair on having the "cojones" to back him."
Sir Christopher said the former Labour leader's "unquestioning support" for the president "eliminated what should have been salutary British influence over American decision-making".
He wrote: "With his Manichean, black and white view of the world, Mr Blair was in his way more neo-con than the neo-cons, more evangelical than the American Christian Right. From this flowed Britain's contribution to the mistakes made before and after the Iraq invasion, despite repeated warnings from the Foreign Office and the Washington embassy."
He added: "The failure to plan meticulously for Saddam's aftermath led to almost a decade of violent chaos and the ultimate humiliation of British forces. Mr Blair's unquestioning support for Mr Bush eliminated what should have been salutary British influence over American decision-making."
Stephen Hadley, Mr Bush's deputy national security adviser, said Mr Blair made clear he would support military action at a meeting at the president's ranch in Crawford, Texas, in April 2002, when the two leaders were left alone for long periods.
He told the Sunday Telegraph: "Mr Blair said that if it came to it, then at the end of the day, he would be with us if we had to move militarily against Saddam Hussein".
Sir Christopher said it was still unclear when the "point of no return" was reached but suggested it was after the the infamous meeting.
"To this day, we do not know exactly what they decided or discussed. I do not believe that at Crawford an irrevocable decision to go to war was taken.
"That was much later, towards the end of the year. But that weekend Mr Blair, for the first time, gave his public support to regime change."