The prime minister is elected like any other MP. At every general election there is a possibility, however remote, that the incumbent governing party could remain the largest in the House of Commons but that their leader would not be returned to Parliament.
No incumbent prime minister has ever lost his or her seat at a general election.
Of recent prime ministers, Boris Johnson’s majority of 5,034 in Uxbridge and South Ruislip is the smallest. Even at its lowest point, Margaret Thatcher’s relatively small numerical majority in Finchley translated into a 20-point lead over the Labour candidate in the constituency. In Uxbridge and South Ruislip, the 2017 general election produced a 10-point gap between Johnson and his Labour rival.
Two prime ministers have come close to losing their seats. In December 1905, Arthur Balfour resigned as prime minister in an attempt to force an election, but the leader of the opposition, Henry Campbell-Bannerman, instead formed a government and became PM. Balfour went on to lose his constituency at the election a month later. In the 1935 general election, Ramsay MacDonald was defeated, having resigned as head of the national government not long before the campaign started.
Balfour was technically the first leader of the opposition in the 20th century to lose his seat at a general election. Herbert Asquith was the second – he was defeated in 1918 and again in 1924 (having returned to Parliament in the interim via a by-election in Paisley). At the 1931 election, Arthur Henderson, leader of the Labour Party, lost his seat during the landslide victory of the national government led by his former party colleague Ramsay MacDonald. Since then, no leader of the opposition has ever lost their seat in a general election.
Leaders of smaller parties have lost their parliamentary seats 18 times since the turn of the 20th century. The Liberal Party lost seven leaders between 1918 and 1979. The most recent example of the phenomenon came in 2010, when the leader of the Democratic Unionist Party (DUP), Peter Robinson, failed to win re-election in Belfast East.
There are three factors to consider when establishing what might happen if a prime minister lost their seat:
- Can they remain as party leader?
- Does a PM need to be a party leader?
- Does the PM need to be an MP?
The Conservative Party constitution says that the leader of the party "shall be drawn from those elected to Parliament". Clause VII of the Labour Rule Book also says its leader "shall be elected from among Commons members of the PLP [Parliamentary Labour Party]." Neither says explicitly that a leader who was no longer an MP would have to resign.
How party leadership rules are interpreted might depend on the level of support the defeated prime minister retained among their MPs and party membership. If a prime minister resigned as party leader, their party would have to organise a leadership election according to their own rules (unless, in the Conservative Party, the 1922 Committee of backbench MPs determined there was only one candidate).
Among smaller parties, it is quite common for party leaders not to sit in the House of Commons. The leaders of the Scottish National Party, Plaid Cymru, DUP, Social Democratic and Labour Party, Ulster Unionist Party and Sinn Féin are all elected in their respective devolved institutions. The Green Party’s two co-leaders are not elected MPs.
The Cabinet Manual says that a PM "will normally be the accepted leader of a political party that commands the majority of the House of Commons", but it does not say that this must be the case.
The process of appointing the prime minister assumes that her or she will sit in the Commons, but there is nothing that says what happens if they cease to be an MP. The prime minister is the Queen’s minister. Precedent suggests that a prime minister should be an MP, but there is no suggestion that they must immediately resign if they lost their seat.
The Cabinet Manual, states that the prime minister “always sits in the House of Commons.” However, this mostly relates to the question of whether they should sit in the Commons rather than the House the Lords.
Although prime ministers regularly sat in the House of Lords in the 18th and 19th centuries, governing from the Commons has been convention since 1902. In 1963 Alec Douglas-Home resigned his peerage and entered the Commons via a by-election when he became leader of the Conservatives.
As no incumbent prime minister has ever lost their seat at a general election, other ministers are the best source of constitutional precedent. It is rare for ministers, particularly Cabinet ministers, to lose their seats, with only 12 serving Cabinet ministers losing their seats since 1974. On 11 occasions, this happened during elections that produced a change of government with Chris Patten, in 1992, the exception. When the election has a clear outcome, defeated ministers resign immediately.
In the early days of a hung parliament, ministers do stay in office without being MPs. In 2010, Jim Knight, then minister of state for employment and welfare reform at the Department for Work and Pensions, lost his seat at the election, but stayed on at DWP until the new government was formed. Similarly, after the general election of 1974, Gordon Campbell continued to serve for a week as secretary of state for Scotland despite losing his Moray and Nairn seat. If the 2019 election leads to a hung parliament, several ministers could face this scenario.
During election campaigns there are no MPs because Parliament is dissolved but government ministers, including the prime minister, remain in place.
The PM’s constitutional status is similar to other ministers, which suggests that he or she could also remain in office temporarily if they lost their seat.
There is no mandated order of succession for prime ministers – the position is dependent on the confidence of the House of Commons and appointment by the Queen. If a PM resigned, having lost his or her seat, another PM would need to take over immediately. This could be an interim figure, in post until a leadership contest had been held by the governing party.
However, in a minority government, recommending a prospective prime minister to the Palace would be dependent on whether that figure could command the confidence of MPs.
In the event that a Labour prime minister lost his or her seat but the party won a majority, the deputy leader would take their place in Number 10 until a leadership election could be organised. When Labour leader John Smith died in May 1994, deputy leader Margaret Beckett served as leader of the opposition until Tony Blair’s election on 21 July 1994.
In the Conservative Party there is no official second-in-command position. If the party held a majority, the Cabinet could nominate an interim prime minister, then its MPs could choose a permanent successor in a leadership contest. If MPs rallied around a single candidate in a leadership contest, then that person could take over very quickly. If the Conservatives did not have a majority, it is much more complicated; it would depend on whether that successor is likely to be able to command confidence.
A prime minister could re-enter the Commons via a by-election, but this would depend on a seat becoming available and would take a minimum of six weeks. It would depend whether the party’s rules could countenance having a leader who was not in the Commons for that amount of time.
There is some precedent. Until the 1926 Re-election of Ministers Act, any MP appointed to a ministerial position at any time other than a general election had to call a by-election to confirm their new position. By-elections have also been used by defeated former prime ministers – after ex-PM Ramsay MacDonald lost his seat in October 1935, he stood for the Combined Scottish Universities seat (abolished in 1950) and returned to the House of Commons in January 1936. He was Lord President of the Council between June 1935 and his retirement in May 1937, remaining in post even in the interim period between his defeat in Seaham and his return to Parliament three months later.
If the election did not result in a clear majority then the defeat of a prime minister might affect whether their party could form a government. Current constitutional practice favours an incumbent PM, and he or she has the opportunity to test whether they can command confidence through a Queen’s Speech. If another MP from the incumbent party was needed to lead the new government, this incumbency preference would not automatically apply.
On the other hand, constitutional practice around hung parliaments is also a reason why a PM might remain in office for a period despite losing their seat. There must be a prime minister in place, so if no replacement was obvious, the incumbent PM might stay on regardless of not holding a seat in the Commons, while parties establish who could command confidence and form a new government.