"From the very beginning, there was a conviction that Saddam Hussein was a bad person and that he needed to go," Paul O'Neill told the CBS television programme "60 Minutes," in an interview broadcast on Sunday.
"For me, the notion of pre-emption, that the US has the unilateral right to do whatever we decide to do, is a really huge leap," he added.
Bush sacked O'Neill - a former chief executive of aluminum giant Alcoa known for his blunt talk - in December 2002 for publicly doubting the need for the president's sweeping tax cut plans.
The interview came after O'Neill served as the main source for an upcoming book, The Price of Loyalty, which paints an insider's view of the Bush administration.
Speaking to Time magazine, O'Neill said: "In the 23 months I was there, I never saw anything that I would characterise as evidence of weapons of mass destruction.
"To me there is a difference between real evidence and everything else. And I never saw anything in the intelligence that I would characterise as real evidence."
Bush took office in January 2001 - and in his first three months in power, officials were already looking at military options to remove Saddam from power, according to documents that O'Neill and other White House insiders gave author Ron Suskind.
"It was all about finding a way to do it," O'Neill is quoted in the book as saying. "That was the tone of it, the president saying, 'Go find me a way to do this.'"
former Treasury secretary
Officials were looking into including post-war contingencies such as peacekeeping troops, war crimes tribunals and the future of Iraq's oil, according to the documents.
One of the memos, marked "secret," says "Plan for Post-Saddam Iraq," Suskind told "60 Minutes."
A Pentagon document, titled Foreign Suitors For Iraqi Oilfield Contracts, talks about "contractors around the world from... 30, 40 countries and which ones have what intentions on oil in Iraq," according to Suskind.
O'Neill told Suskind he was surprised no one on Bush's national security council - which includes national security adviser Condoleezza Rice, Secretary of State Colin Powell and Defence Secretary Donald Rumsfeld - questioned why Iraq should be invaded.
'Find a way to do it'
"It was all about finding a way to do it," O'Neill is quoted in the book as saying. "That was the tone of it, the president saying, 'Go find me a way to do this'."
In one White House meeting, Bush seemed to waver about going forward with his second round of controversial tax cuts.
"Haven't we already given money to rich people?" Bush uttered, according to Suskind, who uses a nearly verbatim transcript of an economic team meeting as a source.
"Shouldn't we be giving money to the middle?"
White House spokesman Scott McClellan on Friday deflected repeated questions about O'Neill's assertions. "I don't do book reviews," he said.
"I'm not going to respond to a book that's not out yet. I haven't seen him explain those comments. I didn't sit in on those meetings, so I wouldn't be privy to any of that."
Don Evans, Commerce Secretary
Aides have often said Bush sets the tone and the broad principles of his administration's policies, but delegates the details to his top advisers.
In a CNN interview on Sunday, Commerce Secretary Don Evans defended Bush, describing him as a decisive president - "focused on the issue, where he is driving the discussion, where he is driving the debate, he is asking the tough questions and then making the tough decisions, and doing it in a very decisive kind of way."
"I'm not going to respond to a book that's not out yet. I haven't seen him explain those comments. I didn't sit in on those meetings, so I wouldn't be privy to any of that," said Evans of meetings O'Neill refers to.
O'Neill was the first cabinet member to leave since Bush took office in January 2001. Environmental Protection Agency head, Christine Whitman, left in June 2003. Housing and Urban Development Secretary, Mel Martinez, quit in December 2003.
In the interview, O'Neill said he was surprised at the lack of dialogue between Bush and his top aides, either as a group or in face-to-face meetings and that he asked no questions during their first one-on-one meeting with him.
"I went in with a long list of things to talk about and, I thought, to engage (him) on... I was surprised it turned out me talking and the president just listening... It was mostly a monologue," O'Neill said.