10 Downing Street, colloquially known in the United Kingdom as Number 10, is the headquarters of the Government of the United Kingdom and the official residence and office of the First Lord of the Treasury, a post which, for much of the 18th and 19th centuries and invariably since 1905, has been held by the Prime Minister. Situated in Downing Street in the City of Westminster, London, Number 10 is over 300 years old and contains approximately 100 rooms.