democratic change.

"He managed the crisis in a way that almost guaranteed that the use of force would become the only feasible option", while failing to be "open and forthright". History, writes McClellan, would record "the decision to invade Iraq as a serious strategic blunder" because "war should only be waged when necessary, and the Iraq war was not necessary".

McClellan, part of the inner-circle of 'Texas Mafia', had worked for Bush for seven years when he quit in 2006. He says he found Dubya "charming and quick witted" and with a brilliant mind for politics. But he also had a "lack of inquisitiveness and a detrimental resistance to reflection" that left him making "gut decisions". Bush was too "insecure" to ever admit mistakes.

It was Bush's ducking the issue of his earlier cocaine habit which first convinced McClellan that his boss manipulated the truth for "political convenience". He believes Bush said something that "probably was not true" when he said that he had been to some "pretty wild parties" but just did not

remember using cocaine.

"How can someone simply not remember whether or not they used an illegal substance like cocaine?" writes McClellan. "It didn't make a lot of sense."

For McClellan the "defining moment" came when he discovered that his denials from the podium in the scandal over CIA officer Valerie Plame's identity were lies. "I had unknowingly passed along false information," he writes. "And five of the highest-ranking officials in the administration were involved in my doing so." They included "the president himself".

The book was to be published on Sunday but was released to reporters in advance by a bookshop in Washington. It is already at the top of the Amazon bestseller list. The White House has denounced it as the ravings of a bitter former employee, while Democratic frontrunner Barack Obama said on his campaign plane that "the only news is that somebody within the administration has confirmed what a lot of us have thought for some time".

McClellan writes of Bush: "Never explaining, never apologising, never retreating... never reflecting, never reconsidering, never compromising."