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The office of Prime Minister of the United Kingdom has evolved over three centuries and is rarely found in statute. “It is impossible to point to a single point in history when the post was created,” observed the Political and Constitutional Reform Committee in 2014, “or even a decision to create it.”

Professor Kevin Theakston and Dr Timothy Heppell have observed that there is “no constitutional definition of the British Prime Minister’s role or any authoritative specification of the office’s functions, powers and responsibilities”. The Cabinet Manual simply notes that the Prime Minister is the head of the UK Government but possesses “few statutory functions”.

In Walter Bagehot’s terminology, while the King heads the “dignified” part of the UK’s uncodified constitution, the Prime Minister leads its “efficient” part, which includes the executive. His or her authority flows from having been chosen by their peers and supporters to lead their party and by the Monarch to lead the government.

Fifty-seven individuals (54 men and 3 women) have served as Prime Minister of the UK, the first and longest serving of whom was Sir Robert Walpole (1721-42). The shortest serving was Liz Truss, who was premier for just seven weeks in 2022.

Although the “Office of the Prime Minister” – or colloquially “Number 10” – is a business unit within the Cabinet Office, there is no dedicated government department which supports the premier.

To Andrew Blick and George Jones, the premiership was “best seen as a cluster of functions, rights and personnel centring on the person occupying the post of Prime Minister”. As Jones put it in 1965:

His office has great potentialities, but the use made of them depends on many variables, the personality, temperament, and ability of the Prime Minister, what he wants to achieve and the methods he uses […] A Prime Minister who can carry his colleagues with him can be in a very powerful position, but he is only as strong as they let him be.

This research briefing first looks at the historical evolution of the role and how a Prime Minister is appointed. It includes a detailed examination of the office’s various functions based on a “cartography” compiled by Professor Lord Hennessy in 2011. He omitted responsibilities as party leader, as does this briefing. Proposals for reforming the office of Prime Minister are summarised, as are the premier’s statutory and prerogative functions.

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