Jack Straw, the Leader of the House of Commons, will today suggest that MPs be given a vote in Parliament in the event of Britain going to war.
Mr Straw will hint in a Fabian Society lecture that the royal prerogative under which a Prime Minister can take Britain to war without a vote should be ended.
In a wide-ranging speech, he will cite the vote before the Iraq war as a precedent for giving MPs a chance to veto military action. However, many MPs will see his speech as a further shift away from Tony Blair towards Gordon Brown, who has also made it clear that he would grant MPs a vote on war in the future.
The vote on war against Iraq was highly unsatisfactory for many MPs who supported the campaign because it came only 24 hours before the US-led forces launched their attacks. Many felt they were faced with a fait accompli and will insist on any future vote being held earlier.
It is the first time Mr Straw has called for a constitutional change to ensure a vote before military action takes place. He will say the vote on the Iraq war eroded the royal prerogative, and was the key to establishing "domestic legitimacy" for the decisions on Iraq.
The former foreign secretary will add: "The process has also established a precedent for the future, making it very likely that any similar decisions about military action would be taken with a parliamentary vote."
His remarks will also be seen as an attempt to regain ground lost to the Tory leader, David Cameron, who pledged to reform the royal prerogative in a speech on the constitution in February. Kenneth Clarke, the former chancellor, is leading a democracy task force which is studying the issue for the Tories.
Mr Straw will also call for radical reform of party funding, following the sleaze allegations that damaged Labour over the "cash for peerages" row.
He will say that there should be new limits on the amount that political parties spend on elections at local and national level to end the "arms race" in political campaigning.
Mr Straw, who still gets up on an orange box at weekends in his Blackburn constituency to speak in his local high street, will urge his colleagues to focus on community politics to get more members of the public actively engaged in party politics.