Making social science accessible

16 Jan 2024

Politics and Society

John Curtice examines what impact the new constituency boundaries will have on the next UK general election, highlighting that the new boundaries maintain a bias in the Conservatives’ favour.

This year’s general election will be fought on new constituency boundaries. These changes are not before time. In England and Wales, the current boundaries were introduced as long ago as the 2010 election, while in Scotland they date as far back as 2005. That has left plenty of potential scope for population movement to have produced big differences in the size of constituencies. For example, in England, Stoke Central contained just 55,400 voters at the last election, down 6,400 on 2010. Bristol West, in contrast, had 99,250, an increase of 16,500.

Meanwhile, the rules under which the neutral Boundary Commissions (one for each part of the UK) are now operating place much more emphasis on having constituencies of an equal size.

Historically, boundary revisions have tended to favour the Conservatives. For most of the post-war period, Britain’s population has shifted out of the cities and into the suburbs and countryside, and from the North towards the South – in short, from more Labour inclined parts of the country to more Conservative orientated locations. Consequently, the electorate in Conservative voting constituencies has tended to grow more rapidly than that in Labour-held ones, a disparity that each boundary review has tried to correct.

Today sees the publication of estimates of what the outcome of the 2019 election would have been if it had been fought on the new boundaries that will be introduced this year. Prepared for the media by the political scientists, Colin Rallings and Michael Thrasher, these are the figures that will form the baseline for the swings and changes in party support that will be reported by broadcasters and the press on general election night. In the meantime, they enable us now to assess the partisan impact of the latest revision of boundaries.

The most immediate measure of their impact is to compare the number of seats it is estimated would have been won by the parties in December 2019 under the new boundaries with the actual outcome of that election. Table 1 shows that despite the length of time since the last review, it seems set to have a relatively limited impact on the parties’ prospects.

The estimates suggest the Conservatives would have had just seven extra seats, while Labour would have had only two fewer. Boris Johnson would have had a majority of 94 rather than 80.  In fact, proportionately at least, it appears that it is the Liberal Democrats (down three) and Plaid Cymru (down two) who have lost out most with the latest review – both parties are dependent on local concentrations of support that the review has inevitably sometimes fragmented or diluted.

Table 1: Result of 2019 election in seats under old and new boundaries

Old boundaries New boundaries Difference
Conservative 365 372 +7
Labour 203 201 -2
SNP 48 48 0
Liberal Democrats 11 8 -3
Plaid Cymru 4 2 -2
Green 1 1 0
Others (Northern Ireland) 18 18 0


This limited impact is not a surprise. The pattern of population movement in recent years, most notably the growth in the population of London, has not simply been one of a flight from the city to less populated areas. Meanwhile, many of the traditional Labour ‘Red Wall’ seats the Conservatives gained in 2019 had smaller than average electorates. As a result, the average electorate in seats won by the Conservatives in 2019 was only just over 2,400 more than in seats held by Labour, a disparity that has been eliminated by the small shift of seats in the Conservatives’ direction.

However, that does not mean that the electoral system would now have treated the Conservatives and Labour equally at the last election. Rather, the new boundaries maintain and reinforce a substantial bias in the Conservatives’ favour.

The bias that existed in 2019 can be seen if we assume there was a 5.85% swing from Conservative to Labour in each and every constituency as compared with the outcome in 2019 (while leaving the shares of the vote won by other parties unchanged). Such a swing would mean that, with 38.9% of the vote each, the Conservatives and Labour would be tied in terms of their share of the vote across Great Britain as a whole. Yet under this scenario the Conservatives would, on the old boundaries, have won 290 seats, 23 more than Labour’s 267.

Now, rather than being reduced by the new boundaries, that disparity has increased to as much as 50 seats.

This inevitably has implications for the ease with which the Conservatives and Labour can win an overall majority. If the swing from Conservative to Labour were uniform, the Conservatives would on the old boundaries need a lead over Labour of 4.8 points to secure an overall majority. That figure that has now edged down to 3.4 points. Labour, in contrast, would need to be as much as 12.3 points ahead of the Conservatives just to secure an overall Commons majority. Now that figure has risen to as much as 13.7 points. This explains why, despite its large lead in the opinion polls, Labour’s task in winning an overall majority is potentially a formidable one.

So why might the electoral system still be heavily biased against Labour at the next election? The explanation lies in the fact that Labour’s vote is less efficiently distributed across constituencies. In particular, if the two parties have the same share of the vote nationally, Labour would win many more seats than the Conservatives by very large majorities. That disadvantage has not been affected by the boundary review, indeed if anything it has been increased somewhat.

However, there is no guarantee that the geography of party support will be the same at the next election. Last year’s local elections together with some polling suggests that support for the Conservatives is falling more heavily in constituencies where they did better in 2019. Meanwhile, some voters also appear willing to vote tactically for whichever opposition party is best placed to defeat the Conservatives locally. If such patterns are present in the election later this year, an electoral system that last time worked to the Conservatives’ advantage may not necessarily be so benign this time around. Labour, at least, certainly need to hope that that proves to be the case.

By John Curtice, Senior Fellow, UK in a Changing Europe, Senior Research Fellow, National Centre for Social Research, and Professor of Politics, University of Strathclyde.


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