Making social science accessible

11 Nov 2022

Constitution and governance

This explainer outlines what a boundary review is, what Boundary Commissions are and how they work, and what the 2023 Review could mean for the next general election.

The Boundary Commissions of the United Kingdom have now issued their revised proposals for the 2023 Boundary Review, a review of all Parliamentary constituencies in the UK. The 2023 Review now enters its final consultation phase, before the Commissions’ final proposals are presented to Parliament next year.

What is a boundary review?

A boundary review occurs when the Parliamentary constituencies of the UK are reviewed by the Welsh, Scottish, Northern Irish and English Boundary Commissions, to ensure that they contain similar numbers of eligible voters and that their borders align with those of local government as far as possible. In the past, boundaries were reviewed every five years. However, with the passage of the Parliamentary Constituencies Act 2020, this has changed to every eight years from 2023 onwards.

What are the Boundary Commissions, and who are their members?

The four Boundary Commissions of the UK are independent of government, non-political, and funded and sponsored by government departments – the Cabinet Office for the English and Welsh Commissions, and the Northern Ireland Office and Office of the Secretary of State for Scotland for the other two Commissions.

“England is set to gain ten constituencies, whereas Scotland and Wales are set two lose two and eight respectively.”

All are chaired by the Speaker of the House of Commons (though s/he plays no role in boundary reviews), with a High Court judge as deputy chair. The deputy chairs in England and Wales are appointed by the Lord Chancellor (i.e., the Secretary of State for Justice), by the Lord President of the Court of Session in Scotland, and the Lord Chief Justice in Northern Ireland.

There are four members of each Commission: the Speaker, a High Court judge, and two other Commissioners. These other two Commissioners are appointed via an open public appointments selection process.

How are boundary reviews conducted?

Boundary reviews are conducted in accordance with the 1986 Parliamentary Constituencies Act. This act has been amended twice in recent times – in 2011, and in 2020, as outlined in the next section. The Act sets out how constituencies should be drawn, specifying that:

  • There should be 650 Parliamentary constituencies
  • Each constituency should have a number of eligible voters that is within 5% of the ‘Electoral Quota’. This is the average number of eligible voters per constituency across the UK, as of two years and ten months before the review takes place. For example, the Electoral Quota for the 2023 review is 73,393. Therefore, each constituency must have between 69,724 and 77,062 voters.
  • Five constituencies are exempt from this 5% rule. These are mostly island seats with smaller than average populations, or pre-drawn local government boundaries, such as Orkney and Shetland and the Isle of Wight.
  • Exemptions can be granted based on the size and accessibility of a constituency, if changes would disrupt the boundaries of existing constituencies, or if any local ties would be broken by a change to a constituency (e.g., a link to a corresponding local authority). There are also special provisions for Northern Ireland.
  • A constituency cannot be larger than 13,000km2.
  • Public consultations should take place during the boundary review process. MPs and political parties can also make representations throughout the process.
  • The Boundary Commission can recommend new or amended names for constituencies.

Previous changes to the boundary review process

Under the coalition government, legislation was introduced to amend the 1986 Act and the way constituencies were drawn up, as well as shrink the House of Commons to 600 seats. This Act was named the Parliamentary Voting System and Constituencies Act 2011, and it reformed the 1986 Act by:

  • Fixing the number of Parliamentary constituencies at 600.
  • Introducing a single UK-wide Electoral Quota, which would be used to allocate seats to the four nations, as opposed to each country using their own quota.
  • Requiring all seats – apart from the then-four exempt island seats – to have a total electorate within 5% of this Electoral Quota.
  • Requiring the Boundary Commissions to conduct a review of constituency boundaries every five years.
  • Allowing interested parties to comment on review proposals during the Review process.

Two reviews were conducted under these rules; the 2013 review, which was abandoned before completion, and the 2018 review, which was not implemented. In March 2020, the government decided to abandon plans to reduce the number of seats in the Commons.


Nine years later, the Parliamentary Constituencies Act received Royal Assent in December 2020. This Act again amended the 1986 Act, by:

  • Fixing the number of Parliamentary constituencies to 650.
  • Scrapping reforms proposed by the 2018 Review.
  • Committing to a new Boundary Review, to be completed by the 1st of July 2023.
  • Requiring the Boundary Commission to conduct reviews every eight years from 2023, instead of every five.
  • Adding Ynys Môn to the list of ‘island seats’ exempt from the 5% Electoral Quota rule.
  • Having the final recommendations of the Boundary Commission’s Review automatically implemented once the process is complete, and removing the need for a vote in Parliament.

What happens next?

Once the Boundary Commissions have their final recommendations, they submit their proposals to the Speaker of the House of Commons.

The government is then responsible for drawing up a draft Order to amend constituencies and presenting this to the Privy Council for approval. The government cannot amend these recommendations, and Parliament does not get to vote on the proposed boundary changes.

“Implementation is automatic – once approved by the Monarch, the new constituencies come into force when Parliament is dissolved for the next general election.”

The government must submit the Order to Privy Council within four months of receiving the proposals. These amendments are then approved personally by the Monarch.

Implementation is automatic – once approved by the Monarch, the new constituencies come into force when Parliament is dissolved for the next general election. Any elections held before the Order is approved by the Monarch will be held under the current constituency boundaries.

The 2023 Boundary Review

The 2023 Boundary Review started in January 2021, following the passage of the Parliamentary Constituencies Act in December 2020. Its final recommendations are due to be issued to the Speaker by 1 July 2023, and will be based on a UK electorate of 46.7 million people.

The Commissions’ initial proposals were issued in Autumn 2021, followed by public consultations in February and March 2022. Revised proposals were presented in October and November 2022 and are open to public consultation until early December.

Under the revised proposals, the number of Parliamentary seats in England, Scotland and Wales have changed (as seen in Figure 2). England is set to gain ten constituencies, whereas Scotland and Wales are set two lose two and eight respectively.

There has been no change to the number of parliamentary seats in Northern Ireland on this occasion, though a boundary review was conducted there.

Though the Commission doesn’t conduct reviews with electoral outcomes in mind, there has been some discussion of what reforms could mean for UK political parties at the next general election. Analysis suggests that, if the general election of 2019 was rerun using these new boundaries, the Conservative Party would gain a further thirteen seats, whereas Labour would lose eight. The Liberal Democrats and Plaid Cymru would lose three and two seats respectively.

By Sophie Stowers, Researcher at UK in a Changing Europe.


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