Tony Blair has been urged to scrap the government's right to go to war without first seeking a parliamentary vote.
The royal prerogative is outdated and should not be used, peers say
A cross-party committee of peers said the vote on the Iraq war in 2003 had created a benchmark, and future decisions should lie with Parliament.
They want the royal prerogative, which lets ministers start military action without such approval, to be changed.
Committee chairman Lord Holme said it was unacceptable that "King Tony" Blair could put soldiers' lives in jeopardy.
Instead the objectives, legal basis and likely size and duration of any campaign should be put to a vote, the committee said in a report.
Chancellor Gordon Brown has signalled he wants prime ministers to be stripped of the power to send troops to war without a parliamentary vote.
Conservative leader David Cameron has said such a change is needed to restore trust in politics.
But an attempt by former Cabinet minister Clare Short to change the law with a private member's bill failed to progress when then Commons leader Geoff Hoon prevented a vote being taken.
In its report, the House of Lords Constitution Committee said the royal prerogative was outdated and should not be used in a parliamentary democracy.
It recommended that as well as seeking Parliament's backing before going to war, the government should also indicate the aims, legal basis, size and likely duration of a conflict.
The peers said they accepted there might be times when war would have to be declared before a vote in Parliament was possible.
But if that happened, a vote should be arranged at the earliest opportunity, they said.
"There is cross-party consensus that it is time to give Parliament, representing the people, the final say on an issue as vital and serious as the despatch of British soldiers, sailors and airmen to hostile situations," Lord Holme told BBC Radio 4's Today programme.
"The case is really what is appropriate in a 21st Century democracy, and the fact that under this fig leaf of the royal prerogative, we switch from 'King Henry's going to war' to 'King Tony's going to war' really won't do.
"If we are going to send our troops into dangerous situations where they may either be killed or kill other people, it should have the support of the nation demonstrated behind it," he said.
"That is best demonstrated in a democracy by the support of the elected parliament."
Peter Facey, director of democratic reform organisations Charter 88 and the New Politics Network, said he was happy there was now "cross-party agreement" on the issue.
"With the situation in the Middle East looking increasingly unstable, this report comes out at an all-too appropriate time," he added.
"If the government deems military intervention necessary, it must be prepared to recall Parliament and seek approval."
The committee's call comes despite Lord Chancellor Lord Falconer robustly defending the current arrangements when he appeared before the committee.
He said formal mechanisms would not work in the reality of military operations.
But Parliament was not powerless under the current rules as it could scrutinise, question and debate decisions.
"You could not possibly go to war with Parliament against you because it is the embodiment of the people," said Lord Falconer.
General Sir Michael Rose, who commanded UN troops in Bosnia, has suggested soldiers do not want to sacrifice themselves for a cause not fully supported by Parliament.
But Armed Forces Minister Adam Ingram told the peers that troop morale was decided by whether they felt they had the right equipment, mandate and leadership.