London, city, capital of the United Kingdom. It is among the oldest of the world’s great cities—its history spanning nearly two millennia—and one of the most cosmopolitan. By far Britain’s largest metropolis, it is also the country’s economic, transportation, and cultural centre. London is situated in southeastern England, lying astride the River Thames some 50 miles (80 km) upstream from its estuary on the North Sea. In satellite photographs the metropolis can be seen to sit compactly in a Green Belt of open land, with its principal ring highway (the M25 motorway) threaded around it at a radius of about 20 miles (30 km) from the city centre. The growth of the built-up area was halted by strict town planning controls in the mid-1950s.