Labour would lose its seat advantage under proportional voting, new modelling suggests

Labour would win a smaller share of Westminster seats under three proportional voting systems than under first past the post, according to modelling published by the Guardian on 30 September 2026. The comparison, produced with More in Common polling, gives Labour 250 seats under the current system but a seat share much closer to its vote share under each proportional alternative. The figures illustrate what changing the voting system could mean for representation; they are projections, not election results.

How the four voting systems change Labour’s projected share

The Guardian compared first past the post with regional party list, single transferable vote (STV) and the additional member system (AMS). Labour’s projected 250 seats under first past the post would leave it short of the 326 seats needed for a Commons majority. Under regional party list, the model gives Labour 26% of seats on 26% of the vote. It gives Labour 26% of seats on 29% of the vote under AMS, and 27% of seats on 28% of the vote under STV.

No party wins a majority in any of the four scenarios. That limits what the seat changes alone can say about who would govern: parties would need to seek support from others, and the Guardian says some of its scenarios could produce a minority government. Its report identifies groupings involving Labour, the Liberal Democrats, Greens, SNP or Plaid Cymru, as well as a possible Reform UK, Conservative and Restore Britain grouping. Those are possible combinations in the model, not agreements between the parties.

The contrast is sharpest for some smaller parties. The Greens win three seats under first past the post in the modelling but more than 50 under each proportional system, according to the Guardian. Restore Britain goes from one projected seat under the current system to more than 30 in each proportional scenario. More in Common’s separate September first-past-the-post projection likewise puts the Greens on three seats and Restore Britain on one.

What the regional projections show

The Guardian’s comparison draws on recent polling, but its poll average and More in Common’s September projection are different measures. The Guardian reports an average of the latest 10 polls putting Labour at 26%, Reform UK at 23% and the Conservatives at 20%, with the Greens and Liberal Democrats each at 10%. More in Common’s September MRP instead estimates vote shares of 26% for Labour, 24% for Reform, 22% for the Conservatives, 11% for the Liberal Democrats and 8% for the Greens. The numbers should not be treated as a single identical polling result.

More in Common’s first-past-the-post MRP projects 147 seats for Reform, 125 for the Conservatives and 63 for the Liberal Democrats alongside Labour’s 250. It puts the SNP on 27, Plaid Cymru on six, the Greens on three and Restore Britain on one, with nine seats for other candidates. Even within that projection, the outcome is sensitive to relatively small changes in support: More in Common says 302 of the 631 modelled seats would be won by margins below five percentage points.

Why the figures are scenarios, not an election forecast

The voting systems in the Guardian’s exercise do not all use the same constituencies or even the same total number of seats. Its party-list model uses 11 regions; STV uses 52 subregional constituencies; and AMS combines regions with 631 existing constituencies. First past the post, STV and party list each have 631 modelled seats, while AMS has 1,052. Northern Ireland and the Commons Speaker’s Chorley seat are excluded. The Guardian says its boundaries, groupings and totals depend on assumptions about how a future system might be designed.

More in Common describes its MRP as a picture of the electorate’s current fragmentation, rather than a projection of the next general election. Its model shows how many seats could be close, but future polling, turnout, candidates and tactical voting remain uncertain. A change in voting rules could also change how people choose to vote. The Guardian’s proportional seat estimates therefore cannot establish the result of an election held under any of those systems.

The modelling arrives as Prime Minister Andy Burnham has announced a national commission on electoral reform. The Guardian reports that he pledged to put a change to the electoral system in Labour’s next election manifesto. That announcement starts a political process; it does not settle which system, if any, would replace first past the post. The Guardian also notes that a Westminster change could face calls for a referendum, citing voters’ rejection of the alternative vote system in 2011.