Parliamentary constituency boundaries are reviewed periodically by independent Boundary Commissions. This House of Commons Library briefing summarises the current rules and procedures for boundary changes and how they have changed over time. It also gives an overview how the number of MPs has reached the current level of 650.
Instruction to the Public Bill Committee considering the Elections Bill 2021-22
On 16 March 2021, the Home Secretary announced that the Government planned to change the voting system for all Police and Crime Commissioners (PCC), Combined Authority Mayors, and the Mayor of London, from the supplementary vote system to the first past the post system, when parliamentary time allowed. On 15 September 2021, Chloe Smith, Minister for the Constitution and Devolution, announced that the Government’s plans would be taken forward in the Elections Bill 2021-22. She also announced that “the voting system for Local Authority mayors in England, which are also currently on the Supplementary Vote system, will be changed to First Past the Post.”
Such amendments are not within the scope of the Elections Bill, so the Government also tabled a motion for an Instruction to allow the Public Bill Committee appointed to consider the Bill to consider these proposals.
Instruction to the Committee of the Whole House
A note on the Order Paper when a motion for an Instruction to the Committee of the whole House considering the dissolution and Calling of Parliament Bill 2021-22 stated that:
Erskine May describes the procedures for considering motions for an Instruction.
Standing Order No 65 gives a general authority to a committee on a bill to amend the bill (even if it means the bill’s long title needs to be amended), “provided that the amendments are relevant to the subject matter of the bill”.
Instructions are required when it is desired to make amendments which fall outside the scope of the bill.
An Instruction either empowers a committee to do something which it could not otherwise do (permissive) or define a course of action it must follow (mandatory).
Permissive instructions may be given to a committee of the whole House or any other committee. Mandatory instructions may only be given to a select committee or to a committee on a private bill. 
Because the question of first past the post voting systems is not currently within the scope of the Bill, members of the public bill committee taking oral evidence on the Bill were not able to ask questions about voting systems. This was outlined in an exchange in the Public Bill Committee sitting on 16 September 2021:
Cat Smith: ….Would it be in order to ask questions of Dr Renwick about electoral systems, given that they are not currently in the scope of the Bill?
The Chair [Christina Rees]: My understanding is that matter is not currently in the scope of the Bill. I am aware that the motion is on the Order Paper for Monday.
Cat Smith: Further to that point of order, is it possible for the Committee to take evidence on electoral systems at any future scheduled evidence sessions that would take place after Monday, when such systems presumably would become part of the Bill?
The Chair: If it is possible to have a supplementary programme motion, then that could be added, but that is not a matter for me. That is usually done through the usual channels.
The Supplementary Vote system
The Supplementary Vote system is used to elect the Mayor of London; directly-elected mayors of local authorities; Police and Crime Commissioners; and ‘metro-mayors’ in combined authorities. These positions were originally established respectively in May 2000; May 2002 (various local authorities); May 2011; and May 2017 (six metro-mayoralties). Up to 2021, elections to these positions have always used the Supplementary Vote system.
The Supplementary Vote system requires electors to vote using two crosses: one in a column for the voter’s first choice candidate, and one in a column for the second choice candidate. The first choices are counted, and the two candidates with the most first-choice votes go through to a run-off. Then, the second choice votes for other candidates are examined, and where those voters have voted for one of the top two as their second choice, their vote is reallocated. The candidate with the largest number of votes following this process is the winner.
A candidate who finished second after the first-choice votes are counted could, therefore, attract enough ‘transfer’ votes to overhaul the candidate who finished first after the first round of voting
If one candidate attracts more than 50% of the first-choice votes, that candidate wins and no run-off takes place (as it would be impossible for the second-placed candidate to overhaul them via second-choice votes). If only two candidates stand in a Supplementary Vote election then the First Past the Post system is used.
It has been comparatively rare, within the Supplementary Vote system, for a candidate to finish second in the first round of voting and to win the election after the second round. This has not happened in six elections for the Mayor of London; it has happened once in 15 metro-mayor elections (Nik Johnson, Cambridgeshire and Peterborough, 2021); and 10 times in 120 PCC elections (eight times in 2012; none in 2016; and twice in 2021).
The Conservative Party’s 2017 manifesto stated “We will retain the first past the post system of voting for parliamentary elections and extend this system to police and crime commissioner and mayoral elections”. The issue is not mentioned directly in the 2019 manifesto, which says “we will strengthen the accountability of elected Police and Crime Commissioners”; and “we will continue to support the First Past the Post system of voting, as it allows voters to kick out politicians who don’t deliver, both locally and nationally”.
In recent Parliamentary debates, the Government has reiterated its commitment to First Past the Post. The most recent Parliamentary Question covering the subject was answered on 3 December 2020:
Baroness Bennett: To ask Her Majesty’s Government what assessment they have made, if any, of plans by the Welsh Government to allow councils to adopt the Single Transferable Vote system for local government elections; and whether they intend to introduce such powers for English councils.
Lord Greenhalgh: The Government does not intend to introduce such powers for English councils. This Government has made a commitment to continue to support the First Past the Post system of voting on a national and local level.
In a debate on electoral reform on 8 June 2020, the Cabinet Office minister, Chloe Smith, said:
The second important feature [of first past the post] is accountability, which with first past the post is linked to place. It can be achieved in other ways, but with first past the post we all get the kind of accountability that comes when an MP walks down the street and looks into the eyes of their constituents. It is important to seek a system that has that in its design, so that there is not some relatively difficult to understand method of apportioning the number of votes, but instead a clear method of who came first—the clue is in the name. That gives citizens, voters, a clear method of holding somebody to account.
The Electoral Reform Society has opposed the proposed changes, in a press release on 16 September 2021:
The system prevents widely unpopular or extreme candidates from slipping in on a fraction of the vote, despite being opposed by the majority.
First Past the Post has seen MPs elected on levels of support as low as 25%. Lowering the bar for winning does nothing to improve the legitimacy of the eventual winner.
A recent analysis from the Electoral Reform Society compared the proportion of votes received by the winner of PCC elections held under the Supplementary Vote system to the proportion of votes that the winner would have gained had the elections been held under First Past the Post.
The Government’s September press release announcing the change said that voters found the Supplementary Vote system confusing. It cited the London mayoral election of 2021, which saw 114,000 spoiled ballots. Details are available from the London Elects website.
 Conservative Party, Forward Together: Our Plan for a Stronger Britain and a Prosperous Future, 2017, p43
 Ibid., p48
 Josiah Mortimer, “Ministers just snuck out a serious, undemocratic change to our voting system in England and Wales”, Electoral Reform Society, 16 Sep 2021
 Michela Palese, “Police and Crime Commissioners and the Supplementary Vote”, Electoral Reform Society, 19 Aug 2021
 See also commentary on this issue from Callum Cuddeford, “Confusion or protest? Record numbers spoil their London Mayor ballots”, SWLondoner, 17 May 2021
On 19 October 2021, the House of Commons will consider a motion to renew the temporary provisions of the Coronavirus Act 2020. The motion is “That the temporary provisions of the Coronavirus Act 2020 should not yet expire”.
Constituencies are reviewed periodically by independent Boundary Commissions, one for each part of the UK. This briefing outlines how the public can get involved in the consultation stages.