US President George W Bush decided to invade Iraq without consulting Vice-President Dick Cheney, Secretary of Defence Donald Rumsfeld or Secretary of State Colin Powell, according to a new book by Washington Post journalist Bob Woodward.

In fact, Powell was not even told until after the Saudi Ambassador to the US, Prince Bandar bin Sultan, was allowed to review war plans in an effort to enlist his country's support for the invasion, according to Plan of Attack, which is being excerpted this week in the Post.

The book is based on interviews with 75 people involved in planning for the war, including Bush.

Woodward, who has written or co-written several best-selling books on US politics, including All the President's Men, quotes Bush as saying he did not feel the need to consult his advisors because "I could tell what they thought".

"I didn't need to ask them their opinion about Saddam Hussein. If you were sitting where I sit, you could be pretty clear. I think we've got an environment where people feel free to express themselves," Bush is quoted as saying.

But Bush said he did discuss the issue with National Security Advisor Condoleezza Rice.

About two weeks before deciding to invade Iraq, Central Intelligence Agency Director George Tenet told Bush there was a "slam dunk case" that Saddam Hussein had weapons of mass destruction, according to Woodward.

The book also reports that in the summer of 2002, $700 million was diverted from a congressional appropriation for the war in Afghanistan to develop a war plan for Iraq.

Woodward suggests the diversion may have been illegal, and that Congress was deliberately kept in the dark about what had been done.

The book says Cheney was the "steamrolling force" behind the decision to go to war, supported by Rumsfeld.

Powell opposed it. Powell and Cheney have such a hostile relationship that they barely speak, according to the book.