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Wales rarely makes the international news headlines. However, last week it produced a political earthquake that saw the UK’s ruling Labour Party lose an election in Caerphilly for the first time since 1910.

Labour has dominated Welsh politics for much of the past century, even keeping much of its support there in 2019, when the party lost many parliamentary seats in the so-called red wall in the English North and Midlands. But last Thursday’s Caerphilly by-election for the Senedd (the Welsh legislature in Cardiff), triggered by the death of a Labour legislator, saw the left-of-center nationalist Plaid Cymru party win 47 percent of the vote. In second place was the right-wing populist Reform UK, led by Brexiteer Nigel Farage, with 36 percent.

The three parties that have governed in the UK Parliament in Westminster since the 1850s secured less than 15 percent of the vote. Labour won 11 percent, the Conservatives 2 percent and the Liberal Democrats only 1.5 percent.

The reason the result could be hugely important is that opinion polls point to growing momentum in Wales for Plaid Cymru and Reform ahead of next May’s Senedd ballots to elect 96 legislators. This would be a pattern-breaker, as Labour has been the largest party in the Cardiff legislature since devolution was initiated more than a quarter of a century ago under Tony Blair’s government.

These developments in Wales are part of a bigger story, in which the British political landscape may be changing faster than at any time in the last century. For more than 100 years, a Labour-Conservative duopoly has tended to dominate UK politics, at least in Westminster. However, that is giving way to a multiparty system that is reminiscent of some continental European democracies. This includes Reform on the populist political right and the Liberal Democrats, the Green Party, the Scottish National Party and the Welsh nationalist Plaid Cymru on the center and left.

One of the defining features of UK politics in 2025 is the unusual, simultaneous rejection of both Labour and the Conservatives. While Labour is suffering from the unpopularity that many incumbent parties are facing across the world, much of the electorate has still not forgiven the Conservatives for their own troubles while in office between 2010 and 2024.

One of the reasons UK politics may be facing a big moment is that 2024 saw the final stage of the exceptionally rare phenomenon of back-to-back general election landslides in opposite directions. The 2019 vote produced a big Conservative win, while in 2024 Labour enjoyed a huge victory. The last time this happened was more than a century ago — 1900 saw a Conservative landslide and 1906 a big Liberal win.

The collective share of the vote for the Conservatives and Labour is among the lowest in living memory. 

Andrew Hammond

The huge victory for the Liberals in 1906 helped catalyze a major political realignment that led to the modern UK electoral system, which has since been dominated by the Conservatives and Labour. Indeed, 1906 was the last time the Liberals won an absolute majority in the House of Commons and was the last general election in which neither Labour nor the Conservatives won the popular vote.

There are today renewed signs of deep-seated disequilibrium in the political system. The collective share of the vote for the Conservatives and Labour is among the lowest in living memory. So, there is a significant possibility of key political change underway. However, rather than giving way to a new two-party system, it might instead be the entrenchment of a multiparty system, heralding a more unpredictable and uncertain political landscape.

However, the decay of the traditional two-party postwar system is not a new phenomenon. Between 1945 and 1970, Labour and the Conservatives collectively averaged in excess of 90 percent of the vote, and also seats won, in the eight UK general elections held in this period. But from 1974 to 2005, their average share of the vote fell significantly. This brought about a significant political change that is, by and large, still unfolding in 2025.

The centrist Liberals have so far done most to break the hold of the two major parties. From 1974 to 2005, their average share of the vote in general elections was almost 20 percent, although the party slumped in the polls after forming a coalition government with the Conservatives from 2010 to 2015.

The apparent decline of the two-party system may make for a more unpredictable outlook for UK politics because it will become harder for any one party to secure a majority in general elections. This is despite the first-past-the-post voting system, which tends to favor the larger parties.

Plaid Cymru’s win in Caerphilly highlights how the UK’s long-standing two-party system appears to be giving way to a much more fragmented political landscape. With polls currently pointing to the SNP being the largest party after Scotland’s Holyrood ballot next May and either Plaid Cymru or Reform winning out in the Welsh Senedd poll, 2026 may see this phenomenon continue.

• Andrew Hammond is an associate at LSE IDEAS at the London School of Economics.