Scott McClellan was a member of Mr Bush's inner circle for years
Former White House spokesman Scott McClellan has said US President George W Bush was not "open and forthright" on Iraq and rushed to an unnecessary war.
In a new book, Mr McClellan says Mr Bush relied on a "political propaganda campaign" to sell the war. His handling of Hurricane Katrina is also attacked.
From July 2003 to his resignation in April 2006, Mr McClellan was a loyal defender of the Bush administration.
In response, a White House spokeswoman said Mr McClellan was "disgruntled".
Dana Perino added: "For those of us who fully supported him, before, during and after he was press secretary, we are puzzled. It is sad - this is not the Scott we knew."
Mr McClellan was a long-standing member of Mr Bush's inner circle, having worked for him when he was Texas governor before following him to the White House.
Extracts from the 341-page memoir, to be published on Monday, give an often scathing view of both the president and his highest-ranking aides.
In What Happened: Inside the Bush White House and Washington's Culture of Deception, Mr McClellan describes White House staff as spending much of the first week after Hurricane Katrina in 2005 "in a state of denial".
George Bush thanks Scott McClellan for all his hard work.
"One of the worst disasters in our nation's history became one of the biggest disasters in Bush's presidency," he writes.
"The perception of this catastrophe was made worse by previous decisions President Bush had made, including, first and foremost, the failure to be open and forthright on Iraq and rushing to war with inadequate planning and preparation for its aftermath."
Mr McClellan stops short of saying Mr Bush lied about the reasons for going to war in Iraq.
However, the way the Bush administration managed the Iraq issue "almost guaranteed that the use of force would become the only feasible option", he says.
Quoted by the Washington Post, he writes that "it was all about manipulating sources of public opinion to the president's advantage" - and chides the media for failing to ask searching questions.
"No-one, including me, can know with absolute certainty how the war will be viewed decades from now when we can more fully understand its impact," he says.
"What I do know is that war should only be waged when necessary, and the Iraq war was not necessary."
'Repeat a lie'
Mr McClellan also accuses former senior Bush strategist Karl Rove and Lewis "Scooter" Libby, Vice-President Dick Cheney's former chief of staff, of misleading him about a CIA leak case involving White House staff.
Libby was found guilty last March of obstruction of justice and perjury over the investigation into the unmasking of CIA officer Valerie Plame.
"Rove, Libby, and possibly Vice-President Cheney allowed me, even encouraged me, to repeat a lie" that Libby was not involved, Mr McClellan writes.
In other excerpts quoted by the Washington Post, he describes Mr Bush as "a man of personal charm, wit and enormous political skill" and says he did not set out to engage in "destructive practices" but became caught up in Washington politics.
Mr Rove, speaking on Fox News, where he is now a political commentator, said Mr McClellan should have spoken out sooner if he had concerns about White House policies.
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