Claim: London was the largest city of any in the European Union at one time.

London -LSB- ˈlʌndən -RSB- is the capital and most populous city of England and the United Kingdom . 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 sqmi 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 , See also : Independent city § National capitals . governed by the Mayor of London and the London Assembly.The London Mayor is not to be confused with the Lord Mayor of London who heads the City of London Corporation , which administers the City of London .   London is a leading global city in the arts , commerce , education , entertainment , fashion , finance , healthcare , media , professional services , research and development , tourism , and transportation . It is crowned as the world 's largest financial centre and has the fifth - or sixth-largest metropolitan area GDP in the world.Rankings of cities by metropolitan area GDP can vary as a result of differences in the definition of the boundaries and population sizes of the areas compared , exchange rate fluctuations and the method used to calculate output . London and Paris are of broadly similar size in terms of total economic output which can result in third party sources varying as to which is the fifth-largest city GDP in the world . A report by the McKinsey Global Institute published in 2012 estimated that London had a city GDP of US$ 751.8 billion in 2010 , compared to US$ 764.2 billion for Paris , making them respectively the sixth - and fifth-largest in the world . A report by PricewaterhouseCoopers published in November 2009 estimated that London had a city GDP measured in purchasing power parity of US$ 565 billion in 2008 , compared to US$ 564 billion for Paris , making them respectively the fifth - and sixth-largest in the world . The McKinsey Global Institute study used a metropolitan area with a population of 14.9 million for London compared to 11.8 million for Paris , whilst the PricewaterhouseCoopers study used a metropolitan area with a population of 8.59 million for London compared to 9.92 million for Paris . London is a world cultural capital . It is the world 's most-visited city as measured by international arrivals and has the world 's largest city airport system measured by passenger traffic . London is the world 's leading investment destination , hosting more international retailers and ultra high-net-worth individuals than any other city . London 's universities form the largest concentration of higher education institutes in Europe . In 2012 , London became the first city to have hosted the modern Summer Olympic Games three times .   London has a diverse range of people and cultures , and more than 300 languages are spoken in the region . Its estimated mid-2015 municipal population -LRB- corresponding to Greater London -RRB- was 8,673,713 , the largest of any city in the European Union , and accounting for 12.5 % of the UK population . London 's urban area is the second most populous in the EU , after Paris , with 9,787,426 inhabitants at the 2011 census . The city 's metropolitan area is the most populous in the EU with 13,879,757 inhabitants , while the Greater London Authority states the population of the city-region -LRB- covering a large part of the south east -RRB- as 22.7 million .  London was the world 's most populous city from around 1831 to 1925 .   London contains four World Heritage Sites : the Tower of London ; Kew Gardens ; the site comprising the Palace of Westminster , Westminster Abbey , and St Margaret 's Church ; and the historic settlement of Greenwich -LRB- in which the Royal Observatory , Greenwich marks the Prime Meridian , 0 ° longitude , and GMT -RRB- . Other famous landmarks include Buckingham Palace , the London Eye , Piccadilly Circus , St Paul 's Cathedral , Tower Bridge , Trafalgar Square , and The Shard . London is home to numerous museums , galleries , libraries , sporting events , and other cultural institutions , including the British Museum , National Gallery , Natural History Museum , Tate Modern , British Library , and West End theatres . The London Underground is the oldest underground railway network in the world .