The reforms are expected to involve Mr Brown giving up royal prerogatives traditionally exercised by the Prime Minister, such as the power to declare war without parliamentary approval or to appoint bishops to the Church of England

And it is understood that the House of Commons will be given new powers, including the right for MPs to recall Parliament in an emergency, to hold US-style confirmation hearings for appointees to key public posts and to ratify international treaties.

There may also be a British Bill of Rights, enshrining the civil liberties of UK citizens currently set out in the Human Rights Act and European Convention on Human Rights.

The proposed constitutional changes were the subject of the first full meeting of Mr Brown's Cabinet on Friday, which he made a point of stressing had been a round-table discussion involving all of his senior ministers.

In a barely-veiled dig at his predecessor Tony Blair, who was criticised for making key decisions in private meetings with a handful of advisers before presenting them as a fait accompli to Cabinet, Mr Brown said: "We had a very long discussion at the first Cabinet about the constitution, which involved every single member of the Cabinet.

"This is not what some people have called 'sofa government'. It is Cabinet Government and the Cabinet ministers have got to be directly involved in decisions."

Speaking on BBC1's Sunday AM, Mr Brown refused to discuss the details of his proposals, insisting that they should be outlined to Parliament first.

He revealed that a statement on his plans had been scheduled for Monday, but has been delayed until later in the week because Home Secretary Jacqui Smith will now be addressing MPs on the recent terror attacks on Monday.

But he said that a key principle would be that any moves to tighten security measures in response to the terror threat would be matched by enhanced judicial oversight and parliamentary accountability to protect civil liberties.