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Parliamentary constituency boundaries are reviewed periodically. This is principally so that constituencies are altered occasionally to take account of changes in population over time. Changes are also made to reflect local government boundary changes, so that administrative boundaries coincide as much as possible.

The reviews are undertaken by independent Boundary Commissions and are currently required every five years. There are four Commissions, one each for the four nations of the United Kingdom. The Commissions are independent of Government but must follow the Rules of Redistribution set out in legislation and decided by Parliament.

The Commissions formulate proposals, using the Rules of Redistribution, which are then published for public consultation. There are currently two rounds of consultation on any initial proposals, which include public hearings. The Commission may then alter their recommendations. If they decide to make alterations, then another stage of consultation takes place. A report with the final recommendations must be handed the Government at the end of the review.

Once the Commissions hand over their reports their role in the process ends. The UK Government must then lay a draft Order in Council before Parliament to give effect to the new boundaries “as soon as may be”. Both Houses of Parliament must approve the draft Order.

The House of Commons (Redistribution of Seats) Act 1944 established permanent Commissions, the Rules of Redistribution, and the requirement to undertake regular reviews. The legislation resulted from the recommendations of the Speakers Conference of 1944.

The Rules guaranteed a minimum number of seats for Scotland, Wales and Northern Ireland and instructed the Boundary Commission for England to maintain the number of seats broadly at the existing level. The Rules of Redistribution were amended several times over the years by Parliament, but the process remained broadly the same. The Commissions had to create seats close to the electoral quota but had discretion to deviate based on other considerations, such as local ties and geographical features.

There was a steady increase in the number of Parliamentary constituencies from 625 in 1950, to 659 from 1997. It remained at 659 until 2005.

In 2005 the number dropped following the reduction in the number of Scottish seats, following the re-creation of the Scottish Parliament. The number rose again in 2010 following the implementation of the Fifth Periodical Boundary Review of Parliamentary constituencies. Since the 2010 General Election there have been 650 seats.

Following the 2010 General Election, the Coalition Government announced that it would bring forward a Bill on electoral reform which would make provision for the creation of fewer and more equal sized constituencies. There had been concern since the 1970s that the Rules of Redistribution needed amending to prevent the ever-increasing number of seats and the differential electorates between seats.

The Rules introduced in 2011 were meant to reduce the House of Commons from 650 seats to 600. Constituencies had to be within 5% of the electoral quota with a single quota for the whole of the UK. The requirement to be within 5% of the quota over-rode other considerations.

Four island constituencies were exempt from the constituency size requirements (Orkney and Shetland, Na h-Eileanan an Iar and two seats on the Isle of Wight). It also made changes to other aspects of the process, such as public consultations.

The first review under the new rules was meant to be completed in 2013. The review was abandoned in January 2013 before final recommendations were produced, and a new deadline for final recommendations was set for 1 October 2018. The reports of the Boundary Commissions were handed to the Government in September 2018.

In March 2020 the Government announced that it would not implement the net reduction in the number of seats in the House of Commons to 600 and instead it would introduce a Bill to amend the Rules of Redistribution.

The Parliamentary Constituencies Bill 2019-21 was introduced on 19 May 2020. It had its Second Reading in the House of Commons on 2 June 2020.

If the Bill is passed, the key changes will be as follows:

  • The number of MPs will be fixed at 650 and the 2018 Review will not be implemented.
  • The next review, due to start in 2021, will have to be completed by the Boundary Commissions by 1 July 2023. It will be based on the number of registered electorates as of 1 March 2020;
  • The next review after the 2023 Review will have to be completed by 1 October 2031; with subsequent reviews required to report by the 1 October every eight years thereafter;
  • Recommendations of the Boundary Commissions will be no longer require Parliamentary approval and government ministers will have no power to alter recommendations;
  • The public consultation phase will be amended to allow for public hearings later in the consultation process.
  • The Boundary Commissions will be given more flexibility to use local government and ward boundaries that have yet to come into force.
  • Add a new protected island seat in Wales to cover Ynys Môn. 

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