The Boundary Commission for England published its proposals for new constituency boundaries in June. A consultation ends in August, which will be followed by revised proposals in 2022 and final recommendations in 2023.

This Insight looks at which boundaries might change, and how much of existing constituencies would be transferred into new ones. See our other Insight for analysis on how voter weight would change in different areas as a result of the review.

What are boundary reviews?

Constituency boundaries are reviewed periodically to make sure that constituencies are all roughly a similar size and reflect local ties between areas. The reviews alter constituencies to reflect rising and falling populations, as well as changes in the boundaries of the wards that comprise them.

The current boundaries were designed based on electorates in 2000 and have been in use since 2010. For more information see our briefings on Constituency boundary reviews and the number of MPs and Parliamentary boundary reviews: public consultations.

The new boundary review is more strict than previous ones when looking at how much electorates can vary in each constituency. Constituencies must now be within 5% of the ‘electoral quota’ of 73,393, except for five protected seats. Previously, up to 10% variation had been accepted.

Measuring change to constituency boundaries

We have measured change by looking at the proportion of the population and homes in existing constituencies that move into new constituencies under these proposals (see ‘below for more information on why we’ve chosen this approach).

The table below shows how much existing constituencies are set to change.

  • 71 constituencies would have over 90% of their current population preserved in their “successor” – the proposed constituency that most closely resembles the current one – if these proposals go ahead.
  • 57 existing seats (one in nine) would have less than 60% of their current population transferred to a successor constituency.
  • 72 constituencies – just under one in seven seats – would be either unchanged, almost unchanged, or have more than 99% of their population retained in a single successor constituency.

Data table showing the number of current constituencies whose boundaries would be affected in different ways by the boundary review proposals. 50 seats would be unchanged, 10 would be almost identical, and 12 would have over 99% of their population preserved. 47 seats would be an enlarged version of their current boundaries, while 67 would be a smaller version of their current boundaries.

Examples of change

For constituencies with ‘boundaries unchanged’, no change is proposed in this review. ‘Almost identical’ covers seats where differences between current and successor constituencies are miniscule, often resulting from changes in ward or local authority boundaries. The example below shows the border between North East Hertfordshire and Stevenage on the outskirts of Stevenage town.

Geographical map showing an example of a very small change between current boundaries and proposed boundaries. A small sliver of land in Hertfordshire is transferred between constituencies.

Enlarged version of old constituency’ covers cases where all the current constituency’s area is included in its successor, but the successor also covers extra areas.

These seats, like Blackpool South shown in the map below, would mostly have been too small to meet the minimum electorate threshold in their current form. This category doesn’t include constituencies which gained new areas but also lost areas elsewhere – these would be covered in the percentage categories. Note that this map and the others below “full extent” constituency boundaries which extend beyond the coastline to the low water mark (low tide).

Geographical map showing an example of a constituency that would be larger after the boundary review proposals. Blackpool South would expand into an area north of its current area.

Smaller version of old constituency’ means that the area covered by the proposed new constituency is a smaller version of the original. These seats, such as Dartford (shown below), would mostly have had electorates higher than the maximum in their current form.

This category doesn’t include constituencies which lost some areas but then gained new ones elsewhere – these are in the percentage categories.

Geographical map showing an example of a constituency that would be smaller after the boundary review proposals. Dartford would retain its northern area but lose some areas south of the A2.

The three constituencies bearing the least resemblance to their proposed successors are Wyre and Preston North (30% of the original constituency is preserved), Stone (30%) and Dulwich and West Norwood (33%). The map below shows how Wyre and Preston North would be divided between five proposed constituencies. Each of these five are a close match to an existing constituency, meaning that Wyre and Preston North is effectively abolished and subsumed into the other five constituencies.

Geographical map showing an example of a constituency that would be split between many proposed constituencies. Wyre and Preston North would be divided between five constituencies.

The table below shows the ten most divided constituencies.

Data table showing which constituencies would be most split (in terms of the percentage preserved in their closest successor): Wyre and Preston North 30% Stone 30% Dulwich and West Norwood 33% Brigg and Goole 36% Denton and Reddish 38% Elmet and Rothwell 39% Bury St Edmunds 39% South Leicestershire 39% Buckingham 40% Wirral South 41%

Mapping the changes

The three maps below show which constituencies are unchanged, which are most changed, and which constituencies have grown or shrunk.

Geographical map showing constituencies that would be changed or almost unchanged by the boundary review proposals. See the downloadable data file for the full list of constituencies in these categories.

Geographical map showing constituencies that would be most changed by the boundary review proposals. See the downloadable data file for the full list of constituencies in these categories.

Geographical map showing constituencies that would be enlarged or shrunk by the boundary review proposals. See the downloadable data file for the full list of constituencies in these categories.

Download the data

More detail on individual constituencies is included in the downloadable spreadsheet attached below. For each constituency, it shows how much of the current seat is contained within its successor; and how much of the successor is made up of the current constituency.

This information is presented on two different bases, as described in the technical details below. For example, Wyre and Preston North’s closest successor is Preston. 27% of the current constituency is contained within the new Preston constituency, and the current constituency makes up 24% of the total of the new Preston constituency. Download data in Excel (199 KB)

Our calculations

We have analysed the similarities and differences between current constituency boundaries and proposed constituency boundaries in two different ways.

First, we’ve calculated the approximate population overlaps using the ONS 2011 Census-based output area population-weighted centroids, combined with mid-2019 population estimates. These centroids represent units of around 300 people based on where they live.

However, this is not sufficient to detect all differences between the current and proposed constituencies. Because the centroids are from the 2011 Census, their locations don’t take account of new homes. The largest case is in the current Thornbury and Yate constituency, where the newly-developed Lyde Green area moves to Filton and Bradley Stoke, but is not detectable by output-area centroids because it did not exist in 2011.

Some differences between constituencies are too small to be detected by output area centroids, as they involve units well below 300 people. The North East Hertfordshire example above is one of these.

Because of the problems above, we have also calculated overlaps using the location of residential addresses. These figures differ slightly from the population-based analysis, because the number of residential properties in different areas aren’t perfectly correlated with the population. However, it does allow us to find all differences that output area centroids cannot. There are 30 such cases in England.

We’ve focused on population and premises here rather than electorates (registered voters), which would be the normal way to analyse constituency changes. This is because it’s not possible to calculate electorates for many of the small overlaps between old and new constituencies. This means we can’t give a precise analysis of changes in terms of electorate.

Alternative measurements

An alternative would be to analyse the change in geographical area between old and new constituencies. But this might give a misleading impression of change.

If a geographically large but sparsely-populated ward moves from one constituency to another, this may appear to be a large change, even if it affects relatively few voters. Similarly, if a geographically small but densely-populated ward moves from one constituency to another, this may appear to be a small geographical change, but it could represent a large number of voters.

Neither population nor premises are a perfect guide to the electorate in an area, but they are a closer substitute than geographical area. In addition, Members of Parliament represent all their constituents – not just those eligible to vote for them – so the total size and distribution of a constituency’s population is also relevant.

About the author: Carl Baker is a statistician specialising in health and geography at the House of Commons Library. Elise Uberoi is a statistician specialising in elections and Parliament at the House of Commons Library.

Photo by William Hook on Unsplash