The
United Kingdom's constitution, being
uncodified and largely unwritten, makes no mention of a
prime minister. Though it had
de facto existed for centuries, its first mention in official state documents did not occur until the first decade of the twentieth century. Accordingly, it is often said "not to exist", indeed there are several instances of parliament declaring this to be the case. The prime minister sits in the cabinet solely by virtue of occupying another office, either
First Lord of the Treasury (office in commission), or more rarely
Chancellor of the Exchequer (the last of whom was
Balfour in 1905). However as the government will have to outline its legislative programme to parliament in, for example, the
Speech from the Throne, the speech is sometimes used to test parliamentary support. A defeat of the Speech is taken to mean a
loss of confidence and so requires either a new draft, resignation, or a request for a dissolution of parliament. Until the early 20th century governments when defeated in a general election remained in power until their Speech from the Throne was defeated and then resigned. No government has done so for one hundred years, though
Edward Heath in 1974 did delay his resignation while he explored whether he could form a government with
Liberal party support.
In such systems unwritten (and unenforceable) constitutional conventions often outline the order in which people are asked to form a government. If the prime minister resigns after a general election, the monarch usually asks the leader of the opposition to form a government. Where however a resignation occurs during a parliament session (unless the government has itself collapsed) the monarch will ask another member of the government to form a government. While previously the monarch had some leeway in whom to ask, all British political parties now elect their leaders (until 1965 the
Conservatives chose their leader by informal consultation). The last time the monarch had a choice over the appointment occurred in 1963 when the
Earl of Home was asked to become Prime Minister ahead of
Rab Butler.
During the period between the time it is clear that the incumbent government has been defeated at a general election, and the actual swearing-in of the new prime minister by the monarch, governor-general, or president, that person is referred to as the "prime minister-elect" or "prime minister-designate". Neither term is strictly correct from a constitutional point of view, but they have wide acceptance. In a situation in which a ruling party elects or appoints a new leader, the incoming leader will usually be referred as "prime minister-in-waiting". An example or this situation was in 2016 in the United Kingdom when
Theresa May was elected leader of the
Conservative Party while
David Cameron was still prime minister.