If a party leader loses their seat, they would no longer be a member of parliament. However, whether they would have to resign as party leader would depend on how party leadership rules are interpreted, and might depend on the level of support the defeated leader retained among their MPs and party membership. If a prime minister resigned as party leader, their party would have to organize a leadership election according to their own rules. If a prime minister lost their seat but their party won a majority, the deputy leader would take their place in Number 10 until a leadership election could be organized. In the UK, the only example of a prime minister losing their seat in an election is Arthur Balfour, who lost his seat in Manchester East during the Conservatives landslide defeat in 1906. In Canada, William Lyon Mackenzie King lost his seat in 1925 and 1945, but his party lost their majority on both occasions, and King returned to parliament in a by-election months later so that he could remain Prime Minister.