The UK could not go to war without a vote in Parliament under a law being proposed by ex-minister Clare Short.
MPs backed the Iraq war
Ms Short came third in the ballot for private members bills. Her plan will get debate time but is unlikely to become law without ministers' support.
Tony Blair allowed a vote two days before the start of the Iraq war but could have sent troops into action using the "royal prerogative".
Ms Short's bill would allow action in emergencies - with a vote afterwards.
And she claimed the key decisions on Iraq had been taken on sofas in Downing Street.
"In this day and age, to have the prime minister having the personalised power to sacrifice and take human life and not be properly accountable to Parliament, because long ago the king used to be able to do it, is not an adequate constitutional arrangement," she said.
Ms Short voted for the war but argues the vote came at the last minute, with MPs were misled about the legal position.
She later resigned from the Cabinet criticising policy on Iraq.
The Labour MP said there was massive public support for the change.
The Commons public administration committee last year also said MPs should have a vote before military action or "soon afterwards" in cases where emergency action was needed.
Labour MP and former Army officer Eric Joyce said it was right for there to be a vote if possible.
But he said: "Ultimately, prime ministers and governments have to make decisions in certain circumstances very quickly with a strong imperative on quick action."
During the election campaign, Chancellor Gordon Brown suggested the Iraq war vote had set a precedent.
Mr Brown told the Daily Telegraph: "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."
Mr Blair has suggested the chancellor was simply backing the current position rather than calling for constitutional changes.
Ms Short said she had not spoken to Mr Brown about her planned bill.
She is expected to use a blueprint bill drawn up by democracy campaign group Charter 88.
The group's co-director, Ron Bailey, said: "Going to war is one of the most important decisions a country can take, but our democratically elected Parliament has no formal right to debate the issue...
"This is not compatible with 21st century democracy."