A metropolitan area is generally defined as consisting of an urban area, conurbation or agglomeration, together with the surrounding area to which it is closely economically and socially integrated through commuting.
The European Union's ESPON project has defined a harmonised series of metropolitan areas across Europe, made up of two components: Morphological Urban Areas (MUAs), which are similar to urban areas and form the densely populated urban cores of metropolitan areas; and Functional Urban Areas (FUAs) which form the labour basin surrounding Morphological Urban Areas.
Morphological Urban Areas have been calculated by combining contiguous local administrative units (LAU 2) with population densities greater than 650 inhabitants per square kilometre, with Functional Urban Areas then being calculated by combining surrounding local administrative units where 10% or more of the workforce works within the core Morphological Urban Area.
According to the harmonised European definition, there were eighteen metropolitan areas in the United Kingdom with populations of more than 500,000 at the time of the 2001 census.
This lists UK metropolitan areas defined by ESPON, which excludes combined conurbations such as the Liverpool-Manchester megalopolis, which has a combined population of 5.6m. It also excludes city regions such as those formed in Greater Manchester, Leeds, Liverpool and Sheffield, which are typically areas covered by a combined authority.
There are currently (2011) 46 metropolitan areas in the United Kingdom with a populations in the range 150,000 to 25 million inhabitants, according to the The European Union's ESPON project, with the following population distributions: