The boundary commissions in the United Kingdom are non-departmental public bodies responsible for determining the boundaries of constituencies for elections to the House of Commons. There are four separate boundary commissions:
Each commission comprises four members, three of whom take part in meetings. The Speaker of the House of Commons is ex officio chairman of each of the boundary commissions. However, the Speaker does not play any part in proceedings, and a Justice is appointed to each boundary commission as Deputy Chairman Commissioner.
The boundary commissions are required to apply a set series of rules when devising constituencies.
Firstly, each proposed constituency has to comply with two numerical limits:
There are a small number of exceptions to the numerical limit on electorate which are specified in the legislation:
Having satisfied the electorate and area requirements, each commission can also take into account a number of other factors:
It is evident that the other factors can to an extent be mutually contradictory, and therefore each commission has discretion on how it applies them. In so doing, each commission aims for a consistent approach within a review.
When a commission publishes its proposals for public consultation, the consultation period is specified in that legislation:
It has been normal practice for local government electoral wards to be used as building blocks for constituencies, although there is no legislative requirement to do so. In Scotland, the introduction of multi-member wards in 2007 has made it harder to do so, since these wards each have a large electorate, and therefore a collection of complete wards may not give an electorate that is close to the required average.
The law specifies that the electorate used during a review is the registered electorate at the time of the start of the review, and not the electorate at the end of a review, or the total population.
Boundary changes can have a significant effect on the results of elections, but boundary commissions do not take any account of voting patterns in their deliberations, or consider what the effect of their recommendations on the outcome of an election may be.
Once a commission has completed a review, it submits a report to the appropriate Secretary of State who puts forward legislation to the Parliament implementing the recommendations. Parliament may approve or reject these recommendations, but may not amend them. If Parliament approves the recommendations, then the Sovereign signs an order formalising the boundary changes which are to be used at the next general election. Any by-elections use the pre-existing boundaries.
Although for many years the legislation gave the Secretary of State the power to modify a commission's recommendations, this power was never exercised. This separates boundary making by a combination of structure and convention from those elected from the resulting electoral areas, with the aim to reduce any scope for gerrymandering.
The commissions are currently established under the Parliamentary Constituencies Act 1986, most recently amended by the Parliamentary Voting System and Constituencies Act 2011. They were first established as permanent bodies under the House of Commons (Redistribution of Seats) Act 1944. The 1944 Act was amended in 1947 and then replaced by the House of Commons (Redistribution of Seats) Act 1949. The 1949 Act was amended in 1958 and 1979 and replaced by the 1986 Act; changes in legislation from 1944 to 1986 were generally incremental in nature. The 2011 Act under the Con-LibDem coalition government made substantial changes to the legislation governing constituency boundary reviews.
The Political Parties, Elections and Referendums Act 2000 under PM Tony Blair's government envisaged that the functions of the boundary commissioners would be transferred to the United Kingdom Electoral Commission, but this never transpired: the Local Democracy, Economic Development and Construction Act 2009 repealed the Act of Parliament (of 2000) effective from 1 April 2010.
Customarily, each commission conducted a complete review of all constituencies in its part of the United Kingdom every eight to twelve years. In between these general reviews, the commissions were able to conduct interim reviews of part of their area of responsibility. The interim reviews usually did not yield drastic changes in boundaries, while the general reviews generally did.
The most recent general review in Wales was given effect by an order made in 2006, in England by an Order from 2007 and in Northern Ireland by an Order from 2008, with the new boundaries used for the May 2010 general election. The most recent general review in Scotland was given effect in 2005, and the resulting constituencies were used in the May 2005 general election.
Under the previous rules, the number of constituencies in Great Britain (England, Wales, and Scotland) had to "not be substantially greater or less than 613", of which at least 35 had to be in Wales. The City of London was not to be partitioned and was to be included in a seat that referred to it by name. The Orkney and Shetland Islands were not to be combined with any other areas. Northern Ireland had to have between 16 and 18 constituencies.
Under the earlier legislation, the terms of review were significantly different:
At the 2010 general election there were 533 constituencies in England, 40 constituencies in Wales, 59 constituencies in Scotland and 18 constituencies in Northern Ireland providing a total of 650.
The Sixth Periodic Review of Westminster constituencies was launched on 4 March 2011 by the Boundary Commission for England, the Boundary Commission for Scotland, the Boundary Commission for Wales and the Boundary Commission for Northern Ireland. The Sixth Review would have resulted in 600 constituencies for the United Kingdom Parliament: a reduction from the 650 constituencies in existence at the 2010 general election. The number of constituencies in each of the four home nations is calculated in proportion to the electorate using a formula set out by legislation. For the Sixth Review, the formula specified 502 constituencies in England, 16 constituencies in Northern Ireland, 52 constituencies in Scotland, and 30 constituencies in Wales.
In January 2013, parliamentary opposition to proposed legislative amendments recommended by this Sixth Review meant that the review was unable to be enacted because of a lack of coalition government consensus. Each Commission is required by law to conduct subsequent reviews of all constituencies in its respective part of the United Kingdom at least every five years.
The use of the December 2015 electoral register has been criticized because it came at the end of the move from household voter registration to individual voter registration, which had reduced the size of the electorate. The changes recommended by the 2011 Boundary Commissions (recommenced in 2016) are expected to benefit the Conservatives the most. There is concern smaller regions such as Wales could lose more MPs relative to other parts of the country.
The scope of the boundary commissions' work is limited to areas for election to the UK House of Commons.
Changes to parliamentary boundaries do not themselves impact on which local councils are responsible for any area.
The procedure for reviews of constituencies and regions for the Scottish Parliament is set down by the Scotland Act 1998. That Act specifies that there are 73 constituencies for the Scottish Parliament: the Orkney Islands, the Shetland Islands and 71 others. The Act also specifies that the constituencies are grouped into eight regions to allow the return of list members elected by proportional representation to the parliament. The Boundary Commission for Scotland conducted a review of these boundaries between 2007 and 2010, whose recommendations were implemented from 2011. Since the legislation requires different numbers of constituencies in Scotland for the United Kingdom Parliament and the Scottish Parliament, these two sets of areas do not fit together neatly. Responsibility for Scottish Parliament boundary reviews passed to the Local Government Boundary Commission for Scotland in May 2017.
The Government of Wales Act 2006 specified that the constituencies for the then National Assembly for Wales were to be the same as those for the UK Parliament at Westminster. The Act required the Boundary Commission for Wales to group the constituencies into electoral regions, to allow the return of list members elected by proportional representation to the Assembly. The Boundary Commission for Wales's Fifth General Review resulted in revised Assembly constituencies and electoral regions. The Parliamentary Voting System and Constituencies Act 2011 removed the link with Westminster constituencies, and there is currently no statutory review body for Senedd Cymru constituencies.
The Boundary Commission reported in 2016 proposing to reduce the number of UK Parliament constituencies in Wales to 29, on the basis that all constituencies must have at least 71,031 voters. While the Parliamentary Voting System and Constituencies Act 2011 removed the link between UK Parliament and Senedd seat boundaries, organisations such as the Electoral Reform Society have indicated a preference for coterminosity (meaning the mirroring of seat boundaries in Wales along the lines of the 2016 proposed reforms to the Welsh seats in the UK Parliament).
Section 33 of the Northern Ireland Act 1998 provides that the constituencies for the Northern Ireland Assembly are the same as the constituencies that are used for the United Kingdom Parliament, but there are six members rather than just one elected from each constituency.