Gordon Brown has said he believes that MPs should be allowed to decide whether Britain goes to war in the future.
The chancellor says the Iraq war set a precedent
The chancellor said the precedent set in 2003 - when Parliament was allowed to vote before the Iraq war - should remain in place.
It would change the prime minister's royal prerogative - which allows him to send armed forces into action.
The comments, in an interview with the Daily Telegraph, come at the end of a week dominated by the Iraq war.
Mr Brown's comments come as Labour and the Liberal Democrats seek to turn their attention to plans for the health services.
The Conservatives are concentrating on pushing their key policies.
In the interview, Mr Brown said: "Now that there has been a vote on these issues so clearly and in such controversial circumstances, I think it is unlikely that except in the most exceptional circumstances a government would choose not to have a vote in Parliament.
"I think Tony Blair would join me in saying that, having put this decision to Parliament, people would expect these kinds of decisions to go before Parliament."
The chancellor claimed that the Iraq war set a precedent which should only be broken in exceptional circumstances and that it should become a permanent feature for any future conflict.
According to the Daily Telegraph, Mr Brown wants changes to the constitution to restore faith among disillusioned Labour voters, who worry about the prospect of military action.
The House of Commons voted on the decision to go to war in Iraq in 2003 but Mr Blair has not indicated a change to the current constitution, which allows him to go to war without parliamentary approval.
If a change is approved it would result in a significant shift in the balance of power between the prime minister and Parliament.
The Liberal Democrats say they would welcome any move to ensuring Parliament does have to approve military action.
But Lib Dem leader Charles Kennedy said: "This debate within the Labour Party is all now being conducted not with a view to the outcome of the general election but with a view to the succession of Tony Blair."
It has been a difficult week for Labour with Michael Howard accusing Tony Blair of lying over the war.
After a series of leaks and mounting pressure Downing Street published the full legal advice it received on the Iraq war on Thursday.
The published advice shows that Attorney General Lord Goldsmith told Mr Blair on 7 March 2003 a second UN resolution was the safest legal course.
Ten days later Lord Goldsmith's final advice was published, but included no concerns about the legality of the war.
Mr Blair has defended his decision to go to war and urged people to look at the intelligence about the Iraq before judging whether he is a liar.