The current constituency boundaries have been in use since 2010. Neither of the two previous boundary reviews (in 2013 and 2018) have been adopted and implemented.
In a written statement in March 2020, the government announced that it is going to restart the stalled programme of new boundaries for Westminster constituencies. This new review, which is yet to be legislated for, will keep the number of seats at 650 and not reduce them to 600 as had been planned by David Cameron's coalition government.
But the next boundary review is likely to require new seats to be nearly identical in terms of the number of electors in each seat. The previous review had a very strict limit, and required each new seat to have an electorate which was not further than 5pc lower or higher than the average. This limit is expected to be retained.
The previous legislation also specified that further new boundary reviews would take place every five years. This week's statement changes that period to every eight years, allowing for two Westminster elections to take place using each set of boundaries.
The current boundaries were first used in 2010 (and 2005 in Scotland), and are now getting a little out of date. Regular boundary reviews are an established part of the electoral process and a new review would be due around now in any case. The reviews are conducted by the four independent Boundary Commissions for England, Scotland, Wales and Northern Ireland and involve periods of public consultation.
This page will contain description, analysis and predictions for the new 2023 boundaries as they are proposed by the four Boundary Commissions.
Electoral Calculus has made a initial approximate estimate of the possible effect of these changes.
The average electorate over all seats in December 2019 was 73,211. But this conceals some wide variations. The average electorate of a seat won by Plaid Cymru in Wales was just over 50,000 voters. And the average electorate of a seat won by the Conservatives was over 74,500 voters, which is nearly 50pc larger. The average electorates of Lib Dem, SNP and Labour seats are also lower than average. These facts provide some support for the Conservatives' arguments that the boundaries could be made more fair.
Our approximate calculations suggest that the Conservatives could gain about half-a-dozen seats from a new review, at the expense of Labour and the nationalist parties in Scotland and Wales. This could increase the Conservative parliamentary majority from 80 to 92.
The disparity between seat sizes can also be seen at a regional level.
The table above shows the current average electorates for each of the twelve regions of the country along with their current number of parliamentary seats. If the seats were all standardised to be of equal size then the number of seats would change. The table shows that Wales (particularly), Scotland and the North and West Midlands of England are currently over-represented and need to lose seats. Conversely, London and the South and East of England are under-represented and need more seats. Since the latter group is more Conservative-leaning than the former, the Conservatives tend to benefit from this correction.
Technical note: the above analysis assumes that four island seats (Orkney and Shetland, Western Isles, and two Isle of Wight seats) will be protected and kept as individual seats despite being under-quota. This requirement was in the original legislation.
The current draft legislation, the Parliamentary Constituencies Bill 2019-21 (text) instructs the four Boundary Commissions (one each for England, Scotland, Wales and Northern Ireland) to conduct new parliamentary boundary reviews with a "review date" of 1 December 2020, and to complete the review before 1 July 2023.
The new parliamentary seats will be made up of, as much as possible, by joining together groups of (entire) local authority wards. This makes the local authority wards an important part of the new seat boundary process, so Electoral Calculus needs to use the same wards as the boundary commissions.
An innovation in the bill is that the wards to be used are the "local government boundaries which exist, or are prospective, on the review date". This means that the wards used will be similar to the wards of 2020, but will include wards which will change in the next couple of years.
As a first step, the Electoral Calculus electoral data has been upgraded to work with the wards of 2020. These "Wards 2020" are similar to the earlier "Wards 2019" which were incorporated in April, but include changes to these local authorities: Basingstoke and Deane, Buckinghamshire, Cambridge, Chorley, Halton, Hartlepool, Oxford, Pendle, Rotherham, Salford and Stroud. The new unitary authority of Buckinghamshire abolishes the previous district councils of Aylesbury Vale, Chiltern, South Bucks and Wycombe. There are a total of 303 new local authority wards.
An additional complication is that the local elections of 2020 were postponed for a year due to the coronavirus outbreak. This means that there were no, and never have been, local elections in those new wards of 2020. Electoral Calculus has used models to infer the likely result of earlier local elections in those new wards.
Using those data, it is possible to infer the likely result of the December 2019 general election in every ward of 2020. This will be useful when the Boundary Commissions report, in terms of predicting the political composition of each new seat.
See also the information about the now-abandoned "2018 review", to compare against the 2023 Review.