The modern office of prime minister developed over several centuries. Medieval and early modern monarchs often had chief ministers who wielded vast power—men such as Cardinal Morton in Henry VII's reign, Burghley under Queen Elizabeth, and Buckingham for James I and Charles I. But they depended totally upon the favour of the monarch, as the fate of Wolsey, Thomas Cromwell, and Clarendon demonstrated. The crucial change came after 1688 when it became necessary to summon Parliament every year, and the ability to manage it became a vital political qualification. Robert Harley, later earl of Oxford, in Anne's reign, had some of the attributes of a prime minister, including a keen understanding of the growing power of the press, but the title of first prime minister is usually given to Sir Robert Walpole, though the term was derogatory and he denied it. The subsequent development of the office depended upon the gradual development of party, which limited the king's choice of minister; on the growing complexity of public business, which demanded a co‐ordinating hand; on the slow decline in the influence of the monarch; and on the development of an organized public opinion, expressed through a reformed electoral system, which substituted the choice of the voters for the choice of the monarch.
As the office grew in stature, the prime minister gradually took over many of the powers of the monarch—the granting and timing of a dissolution of Parliament, the appointment and replacement of ministerial colleagues, and, above all, the granting of honours. Monarchs fought rearguard actions and occasionally won successes, but the general drift was against them. Two heavy blows came in quick succession. In 1832 William IV, with great reluctance, agreed to create enough Whig peers if needed to carry Lord Grey's reform bill, thus allowing a vital royal prerogative to fall into the hands of a determined prime minister: three years later, when he dismissed Lord Melbourne, he was obliged to recall him after Peel had failed to win a majority at the general election.
The powers of the prime minister, though not closely defined, are extensive. He appoints all the other ministers, can transfer them to different offices, or dismiss them altogether. He chairs the meetings of the cabinet and appoints ministers to the numerous cabinet committees. Honours, such as knighthoods, peerages, and other decorations, are awarded on his recommendation. As leader of the government, he exercises a general if not always clearly articulated authority over policy.
In recent years it has become fashionable to describe the office of prime minister as presidential. The official doctrine is that the prime minister is simply the first among equals, and the rule of collective responsibility emphasizes the collegial character of the cabinet. Whenever the post is held by a strong prime minister, the assertion that it has become presidential is propounded, and a contrast is drawn between the office in the 19th cent. and today. The comparison has some force. The urgency of many decisions in the modern world, the increased importance of foreign affairs, media emphasis on the personality of the prime minister, have all tended to enhance the office at the expense of departmental ministers. Yet it is easy to exaggerate the change. The gladiatorial contests between Gladstone and Disraeli anticipated the modern concentration on the rival party leaders.