British troops will only be sent to war in future after their deployment has been approved by MPs under moves announced by Jack Straw, the Justice Secretary.

The Prime Minister will lose the unlimited power to order military action in a package of constitutional reforms which also includes the abolition of the ban on protests near Parliament.

Before wars begin, the Government will have to spell out the aims of the military action and give information about its legality.

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The issue was brought into focus by Britain's involvement in the invasion of Iraq five years ago. Although Tony Blair allowed MPs a vote on the eve of war, he was not obliged to do so. Under the plans set out in a White Paper, the only exception would be when secrecy was essential – such as during covert action by special services – or in times of emergency.

Mr Straw said the change would "define a clear role for Parliament in the most critical of all decisions to face a nation, while ensuring that our nation's security is not compromised".

The ban on demonstrations within 1km of Parliament without police permission will be scrapped. It came into disrepute after it was used to arrest anti-war activists who read out the names of servicemen and women killed in Iraq and the organisers of a peace "tea party" in Parliament Square. A Whitehall source said: "The current law just hasn't worked."

The Government will consult MPs over protecting access to Westminster.