The Prime Minister of the United Kingdom is the head of the Government of the United Kingdom, and chairs Cabinet meetings. There is no specific date for when the office of prime minister first appeared, as the role was not created but rather evolved over a period of time through a merger of duties. However, the term was regularly if informally used of Walpole by the 1730s. It was used in the House of Commons as early as 1805, and it was certainly in parliamentary use by the 1880s. In 1905 the post of prime minister was officially given recognition in the order of precedence. Modern historians generally consider Sir Robert Walpole, who led the government of Great Britain for over twenty years from 1721, as the first prime minister. Walpole is also the longest-serving British prime minister by this definition. However, Sir Henry Campbell-Bannerman was the first and Margaret Thatcher the longest-serving prime minister officially referred to as such in the order of precedence. The first to use the title in an official act was Benjamin Disraeli, who signed the Treaty of Berlin as "Prime Minister of her Britannic Majesty" in 1878.