The position of Prime Minister of the United Kingdom was not created as a result of a single action; it evolved slowly and organically over three hundred years due to numerous Acts of Parliament, political developments, and accidents of history.

Foundations For the various personages who presided over the government of England and subsequently Great Britain at the pleasure of the monarch, usually with said monarch's permission, prior to the government under Robert Walpole as prime minister in 1721, see List of English chief ministers.

Revolutionary Settlement

Because the premiership was not intentionally created, there is no exact date when its evolution began. A meaningful starting point, however, is 1688–89 when James II fled England and the Parliament of England confirmed William III and Mary II as joint constitutional monarchs, enacting legislation that limited their authority and that of their successors: the Bill of Rights (1689), the Mutiny Bill (1689), the Triennial Bill (1694), the Treason Act (1696) and the Act of Settlement (1701).Knappen, pp. 448–451 Known collectively as the Revolutionary Settlement, these acts transformed the constitution, shifting the balance of power from the Sovereign to Parliament. Once the office of Prime Minister was created, they also provided the basis for its evolution.

Treasury Bench

The Revolutionary Settlement gave the Commons control over finances and legislation and changed the relationship between the executive and the legislature. For want of money, sovereigns had to summon Parliament annually and could no longer dissolve or prorogue it without its advice and consent. Parliament became a permanent feature of political life.Smith, pp. 371–373 The veto fell into disuse because sovereigns feared that if they denied legislation, Parliament would deny them money. No sovereign has denied royal assent since Queen Anne vetoed the Scottish Militia Bill in 1708; even then, this was only done on the advice of her government,Smith, p. 382 the last true veto being exercised by William III against the Qualifications Bill in 1696.

Treasury officials and other department heads were drawn into Parliament serving as liaisons between it and the sovereign. Ministers had to present the government's policies, and negotiate with Members to gain the support of the majority; they had to explain the government's financial needs, suggest ways of meeting them and give an account of how money had been spent. The Sovereign's representatives attended Commons sessions so regularly that they were given reserved seats at the front, known as the Treasury Bench. This is the beginning of "unity of powers": the sovereign's ministers (the Executive) became leading members of Parliament (the Legislature). Today, the prime minister (First Lord of the Treasury), the chancellor of the Exchequer (responsible for the budget) and other senior members of the Cabinet sit on the Treasury bench and present policies in much the same way ministers did late in the 17th century.

Standing Order 66 After the Revolution, there was a constant threat that non-government members of Parliament would ruin the country's finances by proposing ill-considered money bills. Vying for control to avoid chaos, the Crown's ministers gained an advantage in 1706, when the Commons informally declared, "That this House will receive no petition for any sum of money relating to public Service, but what is recommended from the Crown." On 11 June 1713, this non-binding rule became Standing Order 66: that "the Commons would not vote money for any purpose, except on a motion of a minister of the Crown." Standing Order 66 remains in effect today (though renumbered as no. 48), essentially unchanged for more than three hundred years.Roseveare, p. 80

Empowering ministers with sole financial initiative had an immediate and lasting impact. Apart from achieving its intended purpose – to stabilise the budgetary process – it gave the Crown a leadership role in the Commons; and, the lord treasurer assumed a leading position among ministers.

The power of financial initiative was not, however, absolute. Only ministers might initiate money bills, but Parliament now reviewed and consented to them. Standing Order 66 therefore represents the beginnings of Ministerial responsibility and accountability.Smith, pp. 372–373

The term "Prime Minister" appears at this time as an unofficial title for the leader of the government, usually the head of the Treasury.Dodd, p. 50 There are a few instances of the use of "Prime" or "First" Minister in the 17th century. After the Restoration in 1660, for example, Lord Clarendon was encouraged to assume the title of "First Minister" in the new government rather than accept a specific office. According to the Duke of Ormonde, however, "He (Clarendon) could not consent to enjoy a pension out of the Exchequer under no other title or pretense but being First Minister... [an office] so newly translated out of French into English that it was not enough understood to be liked and everyone would detest it for the burden it was attended with." Jonathan Swift, for example, wrote that in 1713 there had been "those who are now commonly called Prime Minister among us", referring to Sidney Godolphin, 1st Earl of Godolphin and Robert Harley, Queen Anne's Lord Treasurers and chief ministers.Marriott, p. 87 Since 1721, every head of the Sovereign's government – with one exception in the 18th century (William Pitt the Elder) and one in the 19th (Lord Salisbury) – has been First Lord of the Treasury.

Beginnings of the prime minister's party leadership Political parties first appeared during the Exclusion Crisis of 1678–1681. The Whigs, who believed in limited monarchy, wanted to exclude James, Duke of York, from succeeding to the throne because he was a Roman Catholic. The Tories, who believed in the "Divine Right of Kings", defended James's hereditary claim.

Political parties were not well organised or disciplined in the 17th century. They were more like factions, with "members" drifting in and out, collaborating temporarily on issues when it was to their advantage, then disbanding when it was not. A major deterrent to the development of opposing parties was the idea that there could only be one "King's Party" and to oppose it would be disloyal or even treasonous. This idea lingered throughout the 18th century. Nevertheless it became possible at the end of the 17th century to identify Parliaments and Ministries as being either "Whig" or "Tory" in composition.

Cabinet The modern prime minister is also the leader of the Cabinet. A convention of the constitution, the modern Cabinet is a group of ministers who formulate policies. As the political heads of government departments Cabinet Ministers ensure that policies are carried out by permanent civil servants. Although the modern prime minister selects ministers, appointment still rests with the sovereign.Barnett, p. 247 With the prime minister as its leader, the Cabinet forms the executive branch of government.

The term "Cabinet" first appears after the Revolutionary Settlement to describe those ministers who conferred privately with the sovereign. The growth of the Cabinet met with widespread complaint and opposition because its meetings were often held in secret and it excluded the ancient Privy Council (of which the Cabinet is formally a committee) from the sovereign's circle of advisers, reducing it to an honorary body.Dodd, p. 79. In 1691, for example, a Lord protested, that Cabinet-Council' is not a word to be found in our Law-books. We know it not before: we took it for a nick-name. Nothing can fall out more unhappily, than to have a distinction made of the 'Cabinet' and 'Privy-Council' ... If some of the Privy-Council men be trusted, and some not, to whom is a gentleman to apply? Must he ask, "Who is a Cabinet-Counsellor? ... I am sure, these distinctions of some being more trusted than others have given great dissatisfaction." The early Cabinet, like that of today, included the Treasurer and other department heads who sat on the Treasury bench. However, it might also include individuals who were not members of Parliament such as household officers (e.g. the Master of the Horse) and members of the royal family. The exclusion of non-members of Parliament from the Cabinet was essential to the development of ministerial accountability and responsibility.

Both William and Anne appointed and dismissed Cabinet members, attended meetings, made decisions, and followed up on actions. Relieving the Sovereign of these responsibilities and gaining control over the Cabinet's composition was an essential part of evolution of the Premiership. This process began after the Hanoverian Succession. Although George I (1714–1727) attended Cabinet meetings at first, after 1717 he withdrew because he did not speak fluent English and was bored with the discussions. George II (1727–1760) occasionally presided at Cabinet meetings but his successor, George III (1760–1820), is known to have attended only two during his 60-year reign. Thus, the convention that sovereigns do not attend Cabinet meetings was established primarily through royal indifference to the everyday tasks of governance. The prime minister became responsible for calling meetings, presiding, taking notes, and reporting to the Sovereign. These simple executive tasks naturally gave the prime minister ascendancy over his Cabinet colleagues.Smith, p. 383

Although the first three Hanoverians rarely attended Cabinet meetings they insisted on their prerogatives to appoint and dismiss ministers and to direct policy even if from outside the Cabinet. It was not until late in the 18th century that prime ministers gained control over Cabinet composition (see section Emergence of Cabinet Government below).

"One-Party Government" British governments (or ministries) are generally formed by one party. The prime minister and Cabinet are usually all members of the same political party, almost always the one that has a majority of seats in the House of Commons. Coalition governments (a ministry that consists of representatives from two or more parties) and minority governments (a one-party ministry formed by a party that does not command a majority in the Commons) were relatively rare before the 2010 election; between the elections of 2010 and 2019, there was both a coalition and a minority government. "One-party government", as this system is sometimes called, has been the general rule for almost three hundred years.

Early in his reign, William III (1689–1702) preferred "mixed ministries" (or coalitions) consisting of both Tories and Whigs. William thought this composition would dilute the power of any one party and also give him the benefit of differing points of view. However, this approach did not work well because the members could not agree on a leader or on policies, and often worked at odds with each other.

In 1697, William formed a homogeneous Whig ministry. Known as the Junto, this government is often cited as the first true Cabinet because its members were all Whigs, reflecting the majority composition of the Commons.Marriott, pp. 75–76