The Parliament of the United Kingdom currently has 650 parliamentary constituencies across the constituent countries (England, Scotland, Wales and Northern Ireland), each electing a single member of parliament (MP) to the House of Commons by the plurality (first past the post) voting system, ordinarily every five years. Voting last took place in all 650 of those constituencies at the United Kingdom general election on 12 December 2019.
The number of seats rose from 646 to 650 at the 2010 general election after proposals made by the boundary commissions for England, Wales and Northern Ireland (the Fifth Periodic Review of Westminster constituencies) were adopted through statutory instruments. Constituencies in Scotland remained unchanged, as the Boundary Commission for Scotland had completed a review just before the 2005 general election, which had resulted in a reduction of 13 seats.
Primary legislation provides for the independence of the boundary commissions for each of the four parts of the UK; the number of seats for each of the countries; permissible factors to use in departing from any old boundaries; and a strong duty to consult. The Fifth Review was governed by the Parliamentary Constituencies Act 1986. Under the Parliamentary Voting System and Constituencies Act 2011, as amended by the Parliamentary Constituencies Act 2020, the number of MPs is now fixed at 650. The Sainte-Laguë formula method is used to form groups of seats split between the four parts of the United Kingdom and the English regions (as defined by the NUTS 1 statistical regions of England).
The table below gives the number of eligible voters broken down by constituent country, including the average constituency size in each country.
As the number of electors in each constituency is similar, the constituencies themselves vary considerably in area, ranging from Ross, Skye and Lochaber, which occupies a sixth of Scotland, to the densely-populated London constituency of Islington North.
The "Region" of the table refers to the NUTS 1 statistical region of England, which coincides with the former European Parliament constituency in which the constituency was included until 31 January 2020.
As of the 2019 election there are 533 constituencies in England, 40 in Wales, 59 in Scotland and 18 in Northern Ireland.
Following the abandonment of the Sixth Periodic Review (the 2018 review), the Boundary Commission for England formally launched the 2023 Review on 5 January 2021. It is due to be completed by July 2023.
See 2023 Periodic Review of Westminster constituencies for further details.
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