Liam Fox has accused Conservative leadership front-runner Kenneth Clarke of undermining the morale of British troops in Iraq.
Dr Fox poses for the cameras with fiancee Jesme Baird
Mr Clarke has criticised the decision to go to war in Iraq, a stance not shared by many of his fellow Tories.
The shadow foreign secretary, who joined the leadership race on Thursday, said the Tories would be open to attack from Labour if Mr Clarke was leader.
He said most of the shadow cabinet would be at odds with Mr Clarke.
Dr Fox also said the former Chancellor was trying to win the leadership on the back of a "cult of personality".
Polls suggest Mr Clarke is the most popular choice to succeed Michael Howard among Conservative voters.
Mr Clarke, who is MP for Rushcliffe, has suggested that electing him as leader would help the party exploit the difficulties the government was facing over the situation in Iraq, because of his opposition to the war.
But Dr Fox, speaking to Sky News' Adam Boulton, said that it would result in division.
Kenneth Clarke announced his decision to stand last month
"You can imagine the situation, can't you, where most of the front bench would have supported the war and the leader had been opposed to it," he said.
"That would make a pretty easy target.
"The decision to go to war was made...to go back over this time and time again is to detract attention away from the very important task of reconstructing Iraq.
"And I think it also is dangerous because it undermines the morale of our troops."
Dr Fox, a former GP, went on to say that he thought Mr Clarke's lead in the opinion polls was due to his high recognition factor.
He said: "We have to get back...to that sort of mindset that would break away from the cult of personality which in my view has undermined the Blair government and get back to some government of substance."
Both Mr Clarke and Dr Fox have put their names forward formally as candidates for the Tory leadership, following on from Michael Howard's decision to stand down after the party was defeated in May's general election.
Eight MPs have declared their support for Dr Fox's campaign, which is headed by shadow Commons leader Chris Grayling.
One other Tory - former foreign secretary Sir Malcolm Rifkind - has formally announced he intends to stand. He is continuing his attempt to woo Tory activists on a tour of Britain.
Other leading contenders mentioned are David Davis, the shadow home secretary and David Cameron, the shadow education secretary.