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).
When was prime minister first mentioned in a government document in the UK?
first decade of the twentieth century