London is the capital and most populous city of England and the United Kingdom. Standing on the River Thames in the southeast of the island of Great Britain, London has been a major settlement for two millennia. It was founded by the Romans, who named it Londinium. London's ancient core, the City of London, largely retains its 1.12-square-mile (2.9 km2) medieval boundaries. Since at least the 19th century, "London" has also referred to the metropolis around this core, historically split between Middlesex, Essex, Surrey, Kent, and Hertfordshire, which today largely makes up Greater London, governed by the Mayor of London and the London Assembly.