The Conservatives and Liberal Democrats are stepping up the pressure on Tony Blair over the intelligence used to go to war in Iraq.
Butler has not drawn a line under the Iraq intelligence affair
They say the Butler report published last week raises more questions.
Tory leader Michael Howard told a newspaper he would not have voted for war on the basis of the WMD threat if he had known intelligence was flawed.
Labour said Mr Howard was trying to
"deny responsibility" for his role in supporting the war.
The Tory leader stressed he would have still supported going to war by backing a differently worded motion.
Foreign office minister Mike O'Brien told BBC News Mr Howard was using "lawyerly weasel words in order to try to deny some kind of responsibility".
International Development Secretary Hilary Benn said Mr Howard's remarks plumbed "new depths of opportunism" but senior Tories have defended their leader.
Tory deputy Michael Ancram told BBC News that the government had used "the language of certainty" when speaking of the dangers of Iraq's weapons of mass destruction.
He said: "We now know from Butler that there never was that certainty. There were all sorts of caveats and warnings."
The Butler Report found that the intelligence on Iraq's banned weapons - described by Mr Blair as "detailed, extensive and
authoritative" - had in fact been "sporadic and patchy".
Another senior Tory, former Foreign Secretary Sir Malcolm Rifkind, said Mr Blair should resign because he took the UK to war based on false intelligence.
The prime minister only carried the vote authorising him to commit British troops to
action with the support of the Tories after 139 Labour MPs backed a rebel
"If I knew then what I know now, that would have caused a difficulty. I
couldn't have voted for that resolution," Mr Howard said in an interview with
The Sunday Times.
"If you look at the terms of the actual motion put to the House of Commons on
18 March, 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."
Mr Howard qualified his remarks, saying that he still supported the war and
would have voted for a motion authorising military action if it had been worded
The Tory leader will go head-to-head with Mr Blair in a Commons debate on Iraq and the Butler inquiry next week.
Liberal Democrat deputy leader Menzies Campbell called on the government to apologise for the errors highlighted in the Butler report.
He said: "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."