In a speech on the constitution David Cameron will say today that the time has come to give MPs, not the government, the authority to determine whether the Armed Forces go into battle. The Conservative leader will say that the move will help to restore trust in politics, which has been damaged by Tony Blair.

He will announce that his Democracy task force, led by Kenneth Clarke, the former Chancellor, will examine changes to Royal Prerogative Powers to give Parliament a greater role and make government more accountable.

Mr Blair gave MPs a vote in 2003 before committing troops to Iraq, and has said that an important precedent was set. But Mr Cameron will argue that the constitution needs to be changed to guarantee MPs the final say. “I believe that one of the factors causing disillusionment with politics is the decline in the status and power of Parliament,” he will say.

“Just last week, we first heard about the Government’s decision to send 4,000 troops to Afghanistan in the pages of The Sun newspaper. Restoring trust in politics means restoring trust in Parliament and one way to do that is to enhance the role of Parliament in scrutinising the Government’s decisions.”

He will say that he wants a more formal mechanism for consulting MPs on going to war and agreeing international treaties, where there is no formal role for Parliament: “While there was a vote on the decision to go to war in Iraq, albeit very late in the process, there was no vote on the action in Kosovo. So shouldn’t there be a formal process for parliamentary approval?” Labour is likely to accuse Mr Cameron of attempting to steal Gordon Brown’s ideas. The Chancellor made it clear he intended to make constitutional reform one of the defining areas of his leadership if he succeeds Mr Blair, including giving MPs war-making powers.

Mr Cameron may, however, be prepared to go further than Mr Brown. He will say that there should be a formal role for MPs to scrutinise significant public appointments, such as the chief executive of the NHS or the Chairman of the BBC. Big changes to the structure and organisation of government should also require more formal parliamentary scrutiny.

In an attempt to reassure traditionalists in his party, he made it clear that these were the only areas he would wish the Royal Prerogative to change. “Addressing these matters means addressing the issue of the powers of the Royal Prerogative. I am a staunch supporter of our constitutional monarchy and would not want to undermine it in any way. That is why I have explicitly ruled out any changes to the personal prerogative powers, such as the power to dissolve Parliament and appoint the prime minister.

“We Conservatives are not interested in constitutional tinkering or change for change’s sake. We are passionately committed to preserving and enhancing the integrity of the institutions that make this country great and which guarantee our most precious birthright: freedom under the rule of law.”

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