<P> Standing on the River Thames in the south east 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 km ) 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 , a region governed by the Mayor of London and the London Assembly . </P>
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Standing on the River Thames in the south east of the island of Great Britain