MPs have approved moves to give Parliament the final say on whether British troops should be sent to war.
There was a Parliamentary vote before the Iraq war
Previous calls to strip prime ministers of war-making powers have been rebuffed - but Gordon Brown has said he backs giving MPs more say over such things.
And in a government amendment to a Tory motion, ministers agreed Parliament's role needed to be more "explicit". MPs passed the amendment without a vote.
Tories said ministers had bowed to "strength of feeling" in Parliament.
The prime minister can currently go to war without parliamentary approval, using the Royal Prerogative - although there was a vote on the Iraq war.
But reports by two parliamentary committees have called for a rethink - one said the Royal Prerogative was outdated and should not be used in a parliamentary democracy.
In November the government rejected calls to give Parliament "war-making powers" and the final say on the deployment of British troops in conflicts, arguing it needed to be able to respond to conflicts quickly.
But the Conservatives tabled a motion for debate, arguing that parliamentary approval should be required.
The government has since added its own amendment, stating that it was "inconceivable" that any future decision would depart from the precedent set by the Iraq war vote.
However it added: "The time has come for Parliament's role to be made more explicit in approving (or otherwise) decisions of Her Majesty's Government relating to the major or substantial deployment of British forces overseas into actual or potential armed conflict."
It says there is a "paramount need" not to risk the safety of British forces or commanders' operational discretion, but calls for detailed proposals.
Leader of the Commons Jack Straw also told MPs it was time to formalise the precedent set by Iraq and promised consultation on the matter.
"Reforms of the kind set out in our amendment on what should be Parliament's crucial role in approving or withholding consent for the decision on war, are justified in their own right and are further steps on the road to making, in this country, a constitution fit for this century, not for the last," he added.
Shadow foreign secretary William Hague said: "We will now hold them to this commitment, one previously opposed by Tony Blair but which has been wrung from his government in its dying days."
He added that there was "widespread concern that the power of Parliament to influence government decisions has become weaker over the years and that this trend needs to be reversed".
And Conservative former chancellor Kenneth Clarke said the Iraq vote had not been a good precedent.
He said with troops in the field and the decision effectively already made by the prime minister, Parliament had acted "with a gun to its head".
For the Liberal Democrats, Michael Moore said the government's amendment "edged in the right direction" but ministers could not quite bring themselves "to accept wholeheartedly the principle of parliamentary approval for our forces to participate in armed conflict."
MPs later voted against the Conservative motion by 318 to 217 - a majority of 101.
But the government's proposal was approved without a vote.
Mr Brown, the favourite to succeed Mr Blair as prime minister, has indicated he would be in favour of giving Parliament such powers.