Westminster parliamentary constituencies are the areas used to elect Members of Parliament (MPs) to the House of Commons, which is the primary legislative chamber of the UK and is located in Westminster, London.
At the May 2010 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 8 to 12 years in order 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 authorities 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 though 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, but 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.
The 2001 constituencies were used again in 2005, except for changes to the Scottish Westminster Parliamentary Constituencies, which reduced in number from 72 to 59.
The fifth general review was completed in 2007. It has lead to major change in England and Wales, the majority of constituency boundaries used at the May 2005 General Election have changed.
The Westminster Parliamentary Constituencies boundaries are based on the 5th 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, which affected the Welsh Westminster 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.
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 re-assigning of existing electoral wards from one constituency into a neighbouring constituency.
The only exception concerns Derryaghy 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 - Derryaghy (North) and Derryaghy (South), the former 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.
The following products are available through the ONS Geography Customer Services team (see Related links):
names and codes files which include the 9 character codes for the GSS Coding and Naming policy that was implemented on 1 January 2011
a 'best fit' Output Area (OA) to the 2010 Westminster Parliamentary Constituencies. The methodology used to create the OA Lookup is based on the distribution of 2001 Census population that is, where an OA is split the allocation is based on the distribution of 2001 Census population
a 'best fit' Lower Super Output Area (LSOA) to the 2010 Westminster Parliamentary Constituencies is available. OA's are aggregated to LSOA and like with OA's, where the LSOA's are split by boundaries the LSOA is assigned to Westminster Parliamentary Constituencies according to where the greatest portion of the population lies
a look-up of current postcodes to the 2010 Westminster Parliamentary Constituencies is also available on request (for NSPD customers only). The 1 metre 'centroid' grid reference assigned to postcodes by Ordnance Survey is used, via 'point-in-polygon' methodology, to allocate parliamentary constituencies using a digital boundary set
a look-up of the old Westminster Parliamentary Constituencies to the 2010 Westminster Parliamentary Constituencies which include the 9 character codes for the new GSS Coding and Naming policy that will be implemented on 1 January 2011