Michael Howard says he would not have voted for war in Iraq on the basis of its WMD threat had he known of flaws in the intelligence ministers were using.
Michael Howard will face Tony Blair over Iraq next week
The Tory leader's comments to the Sunday Times follow the Butler Report's criticisms of the weapons intelligence.
Mr Howard said he could not have backed the Commons pre-war motion on WMD, but he would have still supported going to war by backing a different motion.
Labour accused Mr Howard of "plumbing new depths of opportunism".
Speaking of the Commons vote last March, he said: "If I knew then what I know now, that would have caused a difficulty."
"I couldn't have voted for that resolution," Mr Howard told the newspaper.
He said the specific motion referred to Iraq's "weapons of mass destruction and long range missiles"
posing "a threat to international security".
"If you look at the terms of the actual motion put to the House of Commons on March 18 (2003), it placed very heavy emphasis on the presence of weapons of mass
destruction in Iraq.
"So I think it is difficult for someone, knowing everything we know now, to have voted for that particular resolution."
The government only managed to carry the vote with the support of the Tories, after a large rebellion by 139 Labour MPs.
Mr Howard will go head-to-head with prime minister Tony Blair in a Commons debate on Iraq and the Butler inquiry next week.
Both the Tories and the Liberal Democrats had called for a full debate on the issue, after Lord Butler criticised the way the intelligence was presented.
Mr Howard supported the war in Iraq both before and after it happened.
In the Sunday Times interview Mr Howard did say he
would have voted for a motion authorising military action if it had been worded differently.
A spokesman for the Labour party said Mr Howard's remarks plumbed new depths of opportunism and hypocrisy.
Another senior Tory, former Foreign Secretary Sir Malcolm Rifkind, has said Mr Blair should resign because he took the UK to war based on false intelligence.
Liberal Democrat leader Charles Kennedy has called for a further inquiry into the political decisions that led to war.
Liberal Democrat deputy leader Menzies Campbell called on the government to apologise for the errors highlighted in the Butler report.
He told BBC One's Breakfast With Frost: "I don't understand why the government doesn't face up to this. Why on earth doesn't someone apologise?
"Why doesn't someone come to the despatch box and say 'look, we got it wrong. We are sorry we got it wrong'?"
He suggested Tony Blair would face pressure to resign if he did not put up a convincing performance in Tuesday's Commons debate.
"The prime minister's position will depend on the extent to which he is able to defend himself credibly in front of the House of Commons."