The Greater London Built-up Area, or Greater London Urban Area, is a conurbation in south-east England that constitutes the continuous urban sprawl of London, and includes surrounding adjacent urban towns as defined by the Office for National Statistics. It is the largest urban area in the United Kingdom with a population of 9,787,426 in 2011.

The Greater London Built-up or Urban Area had a population of 9,787,426 and occupied an area of 1,737.9 square kilometres (671.0 sq mi) at the time of the 2011 census.

It includes most of the London region – omitting most of its woodland; small, buffered districts; the Lee Valley Park; and the two largest sewage treatment works serving London by the River Thames. Outside the region's administrative boundary, it includes contiguous suburban settlements and a few densely populated outliers connected to it by ribbon development. Its outer boundary is constrained by the Metropolitan Green Belt and it is therefore much smaller than the wider metropolitan area of London.^{[citation needed]}

As a selective grouping of relatively low- to mid-density (and some high-density) output areas, each consisting of roughly 120 households, it can be compared to the London region, which covers 1,572 square kilometres (607 sq mi) and contained 8,173,194 residents at the time of the 2011 census.

The built-up area of the Greater London region continues beyond the region's administrative boundary in some places, while stopping short of it in others. For this reason, the density of the Greater London Built-Up Area is 8.3% higher than that of Greater London, the figure for which includes these outlying rural areas (notably in Hillingdon, Enfield, Havering and Bromley). All of both areas are drained ultimately by the River Thames. The area uses around 4 gigawatts of electricity power.

The density gradient of industrialising cities has tended to follow a specific pattern: the density of the centre of the city would rise during urbanisation and the population would remain heavily concentrated in the city centre with a rapid decline in settlement towards the periphery. Then, with continued economic growth and the expanding networks of public transport, people (particularly the middle-class) would then slowly migrate towards the suburbs, gradually softening the population density gradient. This point was generally reached when the city reached a certain stage of economic development. In London, this point was reached in the first half of the nineteenth century, in Paris towards the end of the century and in New York City at the turn of the twentieth.

However, London had been sprawling out of its medieval confines within the City since the eighteenth century, when the city experienced its first great urban surge. Areas to the west of Westminster were increasingly built up for the wealthy, to live in the suburbs of the city.

A dramatic increase in the city's urban sprawl began in the nineteenth century when labourers flocked from the countryside to work in the new factories that were then springing up. Large developments of small terraced houses began to appear and the new public transport systems – (the Tube, buses and trams) – allowed workers to commute into the city daily. Suburban districts also sprung up around the city centre to accommodate those who wanted to escape the squalid conditions of the industrial town.