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The current boundaries were first used in 2010 (and 2005 in Scotland), and are now getting a little out of date.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.
See also the information about the now-abandoned "2018 review", to compare against the 2023 Review.