The royal prerogative is almost entirely controlled through the advice of the PM or his/her cabinet. However in an emergency where there is no adequate precedent in common law, the monarch may exercise the prerogative independently. In effect, the source of the Monarch's authority lies in common law.Where is that laid down? As far as I'm aware her political power is executive but mostly theoretical, as she follows the advice of her Government. She has the power to refuse assent of a bill, to hire and fire a Prime Minister, and to open and shut Parliament. I'm unaware of the Monarch having an explicit, defined legal or political power to hold MPs to account. HM Government is held to account by HM Loyal Opposition; individual MPs are held to account by the Speaker & parliamentary committees, by their parties, and once every five years by their constituents.