Experts in international law have criticised Tory leader Michael Howard's stance on the Iraq war as "unlawful".
Mr Howard said Saddam Hussein was a threat to peace in the region
Mr Howard told a BBC Question Time Special he would have invaded Iraq even if he had known then Saddam Hussein had no weapons of mass destruction (WMD).
He said the "regime change plus" policy was justified because Saddam Hussein was in breach of UN resolutions.
Legal experts say the stance clearly breaks international law. The Tories say Mr Howard has been misconstrued.
Under international law, it is illegal to invade another country for the purposes of regime change.
Speaking on Question Time, Mr Howard said he would still have supported the war in Iraq as "the right thing to do" even with no threat of WMD.
He said: "Saddam Hussein had been in breach of many UN Security Council resolutions, I think he was a threat to the peace of the region and a threat to the wider peace in the world."
Asked if his position amounted to "regime change war", Mr Howard replied: "Well, it's regime change plus because I also mentioned the breach of the UN Security Council resolutions."
Jeffrey Jowell, professor of public law at University College London, told the BBC News website he was "really surprised" by Mr Howard's attitude.
It is "absolutely clear that international law does not permit regime change", Prof Jowell said.
"You cannot just walk into a country whose government you don't like, even though it may constitute a threat to a region," he said.
"International law has been built up painstakingly over the years in order to bring law and order between nations.
"For Michael Howard to just say that he is going to ignore all that and go in and act like an international cowboy without any respect for legality or constitutionality seems to me to be outrageous."
'Just not possible'
Professor Vaughan Lowe, Chichele professor of public international law at All Souls College, Oxford, said Mr Howard's Question Time performance "was not his finest hour".
"Regime change as such is simply not lawful," he said.
Prof Lowe said he agreed with the attorney general's advice of 7 March 2003 that any military action, whether or not there was a second UN resolution, would have to be confined to disarming Iraq - and could not extend to regime change.
Downing Street decided to publish the full legal advice on war in Iraq
"The idea that you can just go [to war] because the government has an appalling human rights record is just not possible," he said.
Philippe Sands QC said: "If he [Mr Howard] was endorsing use of force for regime change, as he seemed to be doing, then he has committed himself and the Conservative Party to a policy which is plainly contrary to international law.
"I was pretty horrified to hear what he had to say on Question Time. He is after all a barrister and a QC - he must know that there would be no support for such an interpretation of international law."
Maurice Mendelson QC, emeritus professor of international law at the University of London, agreed.
"Essentially the UN charter prohibits the unilateral use of force except in self defence and it only permits the use of force if there is authority from the Security Council," he said.
"In the kind of circumstances which Mr Howard appears to be describing - such as a nasty regime which some states but not the security council as a whole consider to be a threat to international peace and security - it would be unlawful for individual states to use unilateral force to try and change that regime."
A spokeswoman for the Conservatives said Mr Howard's remarks had been misconstrued as a change of position.
The Tories had always argued war in Iraq was justified on the grounds of Saddam Hussein's non-compliance with 17 UN resolutions, she said, and had never based their case on WMD.
Any regime change would be a "by product" of going to war on those grounds, and not its initial remit, she said.
Former Conservative foreign secretary Sir Malcolm Rifkind defended Mr Howard's stance as demonstrating his honesty.
The Tories accuse Tony Blair of being a "liar" who misrepresented the basis for going to war.
Mr Blair has insisted the war was lawful and the decision to invade Iraq was made in good faith.
Lib Dem foreign affairs spokesman Sir Menzies Campbell, whose party opposed the war, said Iraq was a credibility problem for the Tories as it was for Labour.