Westminster parliamentary constituencies are the areas used to elect Members of Parliament (MP) to the House of Commons, which is the primary legislative chamber of the UK and is located in Westminster, London.
At the May 2010 General Election, there were 650 constituencies. The number of MPs in each constituent country of the UK is: 533 in England, 59 in Scotland, 40 in Wales and 18 in Northern Ireland.
Boundary Change and Geographic Constitution
Constituency boundaries are determined by the Boundary Commissions (one each for England, Scotland, Wales and Northern Ireland).
The commissions are required to undertake a general review every eight to 12 years to ensure electoral equality – that is, that the sizes of the electorates in each constituency are as similar as possible (currently about 70,000 electors, typically reflecting a total population of 90,000).
General reviews normally lead to large-scale changes, but the commissions may also carry out localised interim reviews. These can occur at any time, but usually only lead to minor changes.
Constituencies are generally based on whole or part local authority districts unless there is a strong case to straddle boundaries – each case is decided on its merits; constituencies may not, however, split electoral wards/divisions.
Note, that although constituencies are defined to reflect wards at the time of review, the changes are not implemented until the subsequent General Election.
Accordingly the constituencies used in the 1997 General Election were mostly based on the April 1994 ward boundaries used for the fourth general review. However, a small number of constituencies used April 1995 boundaries, and some were affected by subsequent interim reviews.
The 1997 constituencies were used again in 2001, except for some mostly minor boundary changes in London and south-east England resulting from interim reviews. In 2005, the boundaries were retained, with the exception of the Scottish Westminster parliamentary constituencies. These were reduced in number from 72 to 59.
The fifth general review was completed in 2007. It led to major change in England and Wales; the majority of constituency boundaries used at the May 2005 General Election were changed.
The Westminster parliamentary constituencies boundaries are based on the fifth Periodical Parliamentary Review that was promulgated in July 2007 (defined in terms of wards as at 12 April 2005).
An amending order in July 2008, affected the Welsh Westminster parliamentary constituency boundaries and a further amendment order (operative 18 March 2009) realigning the boundaries for Daventry, South Northamptonshire, Somerton and Frome and Wells has also been applied.
There will be no changes for the May 2015 General Election.
In Northern Ireland, the commission's Fifth Periodical Report on Parliamentary Constituencies outlined changes to and the composition of constituencies.
In practice, the new parliamentary constituencies are not very different to their predecessors. As before, there are 18 constituencies, all retaining the same name. In the main, the boundary amendment process has involved the reassigning of existing electoral wards from one constituency into a neighbouring constituency.
The only exception concerns Derryiaghy ward within Lisburn City Local Government District, which previously resided entirely within Lagan Valley county constituency. It has now been split into two separate wards – Derryiaghy (North) and Derryaghy (South), the former is now included in Belfast West borough constituency, the latter remaining where it was.
County and Borough Constituencies
Sometimes constituencies are referred to as either borough (burgh in Scotland) or county constituencies. Borough constituencies are predominantly urban whereas county constituencies are partly or mostly rural.
Definitions are allocated by the Boundary Commissions and affect candidates’ election expenses and also who can be the constituency's returning officer.
If used, the designation is suffixed to the constituency name and is generally abbreviated: BC for borough constituency, CC for county constituency.
Several products including names and codes, lookups and boundaries are available to download from the Open Geography portal.