Constituency boundaries after 2015: Key issues for the 2015 Parliament

After the expenses scandal there were calls to cut the cost of politics and one of the proposals was to reduce the size of the House of Commons.

In 2011 legislation was passed to reduce the number of MPs from 650 to 600, but the review of constituency boundaries that would have made the recommendations necessary to implement these changes was halted because of disagreements within the previous Government over constitutional reform.

Under the law as it still stands, a new review by the Boundary Commissions must be completed by October 2018. It must again divide the UK into 600 constituencies.

Divide and rules

The Parliamentary Constituencies Act 1986, as amended, requires the four Boundary Commissions – one for each country of the UK – to keep the boundaries of Parliamentary constituencies under continuous review, and conduct periodic reviews of all constituencies.

In proposing new boundaries, the Commissions do not have free rein: they must operate under the Rules for Redistribution in the Act, which set out, among other conditions, the number of constituencies that there should be, and the extent to which the size of the electorate in each constituency can differ from the electoral quota (average size of a constituency).

The previous Government gave effect to its pledge to create “fewer and more equal sized constituencies” by changing the Rules for Redistribution through the Parliamentary Voting System and Constituencies Act 2011.

The Rules now require that there should be a prescribed number of 600 seats for the whole of the UK following the next general review of Parliamentary constituencies.

Chart 1: Number of MPs and population per MP

A reduction in the number of MPs to 600 would leave the House of Commons smaller than at any time since 1800, and the population per MP higher than ever: Number of MPs (lighter shade, left-hand axis) and population per MP
Chart showing the number of MPs and the population per MP since 1800

The legislation resulted in other changes to the Rules,which broadly reduced the discretion available to the Commissions in drawing up constituency boundaries.

What happened to the Sixth Review?

The Sixth Review came to a halt because of disagreements within the previous Government over constitutional reform.

The Deputy Prime Minister announced on 6 August 2012 that plans to reform the House of Lords were to be dropped and that consequently the Liberal Democrats would not vote to approve the Order implementing the recommendations of the Boundary Commissions.

The Commissions continued their work until early 2013, when the law was changed to allow the review to be delayed by an electoral cycle. The Commissions must now conduct a review after the 2015 general election and submit reports by October 2018, in time for their recommendations to take effect by the next scheduled general election in 2020.

Chart 2: Percentage change in seats following Sixth Periodic Review

Under the Boundary Commissions’ recommendations, some areas would have seen much larger reductions in MPs than others percentage change in seats following Sixth Periodic Review

Chart showing the change in seats by region following the sixt periodic review

The 2018 Review

Had it been implemented, the 2013 Review would have made dramatic changes to many constituency boundaries.

Some argued that these would have been disruptive and administratively clumsy because constituency and local government would no longer have aligned in many areas.

Since the 2018 Review must be conducted under the same Rules, it is unlikely that these concerns can be addressed fully unless primary legislation is passed to change the Rules.

A particularly restrictive element of the existing Rules highlighted in a March 2015 Report of the Political and Constitutional Reform Committee was the requirement that the size of the electorate in all constituencies (with four exceptions) be within 5% of the electoral quota.

The Committee recommended that this constraint be relaxed to 10%. It also reiterated its previous conclusion that the case for reducing the number of MPs from 650 to 600 had not been made.

Given the time required to complete boundary reviews, any changes to the Rules would have to be a priority for the new Government if the October 2018 deadline is to be met.

£12.2 million
The expected annual savings resulting from reducing the number of MPs from 650 to 600.

Party Lines

  • Conservatives: implement the boundary reviews automatically once the commission reports in 2018
  • Liberal democrats: cancel the boundary review
  • UKIP: ensure constituencies across the country are of equal size


Current Briefing Papers

The Key Issues articles were published in May 2015. However, the Commons Library produces new briefings as topics evolve.

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