The prime minister of the United Kingdom exercises functions in both the executive and the legislature, as the UK has a fusion of powers.

Executive powers of the prime minister include obtaining at any time the appointment or dismissal of all other Government ministers, exercising the royal prerogative, setting the Government's policy agenda and priorities, and deploying the British Armed Forces overseas. In the legislature, the prime minister normally (but not necessarily) leads the largest party in the House of Commons, and therefore can normally gain Parliamentary support for their desired legislation. Most of the time the prime minister can control what is debated in the House of Commons, and when, as Government business has precedence (that is, priority) on every day it sits. General elections are called at a time of the prime minister's choosing, with a legal maximum of five years between elections. The prime minister, and other ministers appointed by the prime minister, make many of the most senior Crown appointments. The prime minister also has some power over their own party, in their role of party leader.

The powers of the prime minister come from several sources of the UK constitution, including both statute and constitutional convention, but not one single authoritative document. They have been described as "...problematic to outline definitively."

The status and executive powers of the British prime minister means that the incumbent is consistently ranked as one of the most powerful democratically elected leaders in the world.

Definition of the role and its powers

Between 2011 and 2014, the Political and Constitutional Reform Select Committee conducted an inquiry into the "role and powers of the Prime Minister". In their report, they wrote: When Lord Hennessy gave evidence to us, he commented: "the role of the Prime Minister is like the British constitution as a whole—you think you are getting close and it disappears into the mists." With the role itself difficult to define, it is not surprising that the powers of the person who fulfils the role are similarly problematic to outline definitively. Most powers exercised by the Prime Minister are not defined in statute and cannot be found in one place. In his book The Prime Minister: The office and its holders since 1945, Lord Hennessy stated: "Arguments have raged around the powers of the British Prime Minister for nearly 300 years."

Without a clear definition of the role of prime minister, the powers associated with the office have been able to evolve and accumulate, unhindered by statute, over hundreds of years...

There is no single authoritative source for what the role involves and the powers the prime minister can exercise.

Documents describing the role and powers of the PM include: From 1947 to 1948, the Cabinet Office prepared a paper titled "Function of the Prime Minister and his Staff". In 2002, a private member's bill was drafted to define the "office, role and functions" of the PM, but this was never passed. In 2011, the Cabinet Office published The Cabinet Manual, setting out the main laws, rules and conventions affecting the conduct and operation of government, including a description of the role of the PM. In 2011, Peter Hennessy gave a list of 47 functions, under eight headings, of the prime minister. In 2014, the Political and Constitutional Reform Select Committee produced a final report to their Inquiry, titled Role and powers of the Prime Minister.

Executive powers

Appointing ministers

When commissioned by the sovereign, a new prime minister's first requisite is to "form a Government"—to create a cabinet of ministers that has the support of the House of Commons, of which he is expected to be a member. The prime minister nominates all other cabinet members to the sovereign for appointment (who then, if they are not already, names them Privy Counsellors) and ministers, and although consulting senior ministers on their junior ministers, without any Parliamentary or other control or process over these powers. The prime minister decides the ranking order of all ministers.

The prime minister holds powers of patronage which means that at any time, he may obtain the appointment, dismissal or nominal resignation of any other minister. The prime minister may implement a cabinet reshuffle; one famous example was the Night of the Long Knives in 1962. The prime minister may resign, either purely personally or with the whole government. A prime minister is no longer just "first among equals" in HM Government; although theoretically the Cabinet might still outvote the prime minister, in practice the prime minister progressively entrenches his position by retaining only personal supporters in the Cabinet. In occasional reshuffles, the prime minister can sideline and simply drop from Cabinet the Members who have fallen out of favour; they remain Privy Counsellors, but the prime minister decides which of them are summoned to meetings.

The Ministerial and other Maternity Allowances Act 2021 provides for the prime minister to grant certain office holders six months' maternity leave at full pay. In the case of ministers, the prime minister may designate that person a 'Minister on Leave', which comes with the salary of that person's previous office for up to six months.

The prime minister is responsible for producing and enforcing the Ministerial Code.

Royal prerogative powers

The prime minister formally kisses the hands of the sovereign, whose royal prerogative powers are thereafter exercised solely on the advice of the prime minister and His Majesty's Government ("HMG"). The prime minister has weekly audiences with the sovereign, whose rights are constitutionally limited by convention: "to warn, to encourage, and to be consulted"; the extent of the sovereign's ability to influence the nature of the prime ministerial advice is unknown, but presumably varies depending upon the personal relationship between the sovereign and the prime minister of the day.

As, by constitutional convention, the monarch acts on ministerial advice in all but exceptional cases, it is effectively ministers who exercise the royal prerogative. And it is, in reality, the prime minister who appoints, accepts the resignations of and dismisses ministers. Rodney Brazier has said:Of greatest political importance is the Prime Minister's personal authority, which stems from his ability to use, or to advise the Sovereign to use, certain prerogatives. If this country has prime ministerial government, it is based on the Prime Minister's personal authority which, although political in effect, is rooted in the prerogative.

Government policy

The prime minister sets the Government's policy agenda and priorities, and generally co-ordinates the policies and activities of the Cabinet and Government departments, acting as the main public "face" of His Majesty's Government. The prime minister is ultimately responsible for all policy and decisions.

The prime minister can overrule the policy decisions of individual ministers. Boris Johnson reportedly overruled home secretary Priti Patel on closing UK borders, Tony Blair overruled Scottish secretary Donald Dewar on devolving abortion powers to Scotland, Gordon Brown overruled chancellor Alistair Darling on VAT rates, and Margaret Thatcher overruled home secretary Leon Brittan on parole for Ian Brady and Myra Hindley.

Some policy decisions are made by the Cabinet. The prime minister chairs the Cabinet, decides its membership, and when and where meetings take place. Other important policy decisions are made by the Cabinet Committees. The prime minister is free to decide the Cabinet Committees' structure, membership, chairmanship, and terms of reference. A number of Cabinet Committees are chaired by the PM.

The prime minister is the minister responsible for national security, and matters affecting MI5, MI6 and GCHQ collectively, though other ministers may authorise individual operations. The PM chairs the National Security Council, and appoints the National Security Adviser.

Some Government policy decisions are of such significance that the ultimate decision of how to proceed is made by the prime minister rather than the relevant minister. For example:

In 1842, prime minister Robert Peel re-introduced income tax. In 2006–2007, prime minister Tony Blair decided that the UK should renew Trident. In October 2007, prime minister Gordon Brown approved the construction of Crossrail. In 2016, David Cameron called an EU referendum. In September 2016, Theresa May authorised the building of Hinkley Point C nuclear power station. In 2020–2021, Boris Johnson decided that various restrictions and lockdowns should be imposed in response to the COVID-19 pandemic.

The prime minister might have his own policy agenda which he drives to implement. For example: Margaret Thatcher drove forward a set of policies now known as Thatcherism. Tony Blair led his Government in following a strategy known as the Third Way. David Cameron pushed forward implementation of the Big Society concept. Theresa May sought to expand the number of grammar school places.

Machinery of government

The prime minister has "almost complete discretion over how to organise government departments". The PM is free to, at any time, create government departments, merge them, rename them, transfer responsibilities between them, and abolish them.

The civil service

The prime minister of the day has held the office of Minister for the Civil Service since that office was created in 1968. As such the PM has the powers over His Majesty's Civil Service held by that position. These powers may be delegated by the prime minister to others. This power is under review.

Ministers' advisers The Constitutional Reform and Governance Act 2010 states that a special adviser to a minister in the UK government can only be appointed if the appointment has been approved by the prime minister.

Similarly, the Ministerial Code says that while Cabinet Ministers and Ministers of State may appoint Parliamentary Private Secretaries, all appointments require the prior written approval of the prime minister.

The Armed Forces

The Head of the Armed Forces is the monarch. However, Hennessy says that "war is an intensely prime ministerial activity."

The decision to deploy the armed forces overseas rests with the prime minister or the Cabinet, through the exercise of the royal prerogative. Constitutional convention requires that, in the event of a commitment of the armed forces to military action, authorisation is given by the prime minister, on behalf of the Crown. Decisions on military action are taken within the Cabinet with advice. The Defence Council has power of command over members of the armed forces.

However, in 2011, the government of the United Kingdom acknowledged that a constitutional convention had developed whereby the House of Commons should have an opportunity to debate the matter before troops are committed. It said that it proposed to observe that convention except when there was an emergency and such action would not be appropriate.

Similarly, declarations of war by Great Britain and the United Kingdom are through the exercise of the royal prerogative by ministers, and led by the prime minister.

The prime minister is responsible for the decision to shoot down a hijacked aircraft or an unidentified civil aircraft which responds neither to radio contact nor the signals of RAF interceptor jets, before it reaches a conurbation or a key target on UK territory. The prime minister is also responsible for authorising the use of UK nuclear weapons, including the preparation of four letters of last resort. He also appoint deputies for both of these purposes.

Legislative powers

Primary legislation