Explainer

By-Elections Explained: How They Work, Recent Results & What They Tell Us

By-elections are political laboratories. Free from the pressure of a general election result, voters can send signals, protest, and experiment in ways they rarely do when the national government is at stake. For pollsters and political analysts, they are fascinating — but must be interpreted with extreme caution.

What is a by-election?

A by-election is an election held in a single parliamentary constituency to fill a vacancy between general elections. Vacancies arise from the death of the sitting MP, resignation from Parliament (MPs must apply for a nominal paid office such as Steward of the Chiltern Hundreds to formally resign), disqualification after conviction for a serious offence, elevation to the House of Lords, or a successful recall petition. Since 2015, the Recall of MPs Act has allowed constituents to trigger a by-election if their MP is suspended from Parliament for at least ten sitting days or receives a custodial sentence.

How a by-election is triggered

By convention, the party of the outgoing MP moves the writ — the formal parliamentary order to the returning officer to hold an election. The Speaker then issues the writ. Once issued, the election must be held within 25 working days.

There have been notable exceptions to the convention of prompt writ-moving. Between 2017 and 2019, the Conservative government delayed several writs to avoid contests at disadvantageous moments. This practice is technically within parliamentary rules. The UK Parliament website maintains a complete record of by-election writs and dates.

How by-elections work differently from general elections

By-elections have structural features that make them differ systematically from general elections:

• Much lower turnout: General election turnout is typically 60–70%; by-election turnout often falls to 30–45%. Lower turnout amplifies the voice of more motivated, politically engaged voters and disadvantages the governing party
• Protest voting: Because a single by-election cannot change the government, voters can express dissatisfaction without consequence
• Tactical concentration: Opposition parties concentrate massive campaign resources on a single seat
• Local factors: The cause of the vacancy, local candidate quality, and local issues can dominate in ways they rarely do nationally
• Media attention: Each by-election receives national coverage entirely disproportionate to its parliamentary significance (one seat out of 650)

Historic by-elections and what they signalled

Several by-elections have proven historically significant as leading indicators of political change:

• Orpington 1962: The Liberals overturned a Conservative majority of 14,760, signalling deep mid-term dissatisfaction with Harold Macmillan’s government
• Newbury and Christchurch 1993: The Liberal Democrats took both seats from the Conservatives with swings exceeding 25%, heralding the scale of Conservative unpopularity that delivered Blair’s 1997 landslide. See our results archive for historical context
• Brent East 2003: The Liberal Democrats gained Brent East from Labour on a 29% swing, driven substantially by opposition to the Iraq War
• Glasgow East 2008: The SNP gained this safe Labour seat on a 22% swing, heralding the coming SNP dominance of Scottish politics
• Clacton 2014: Douglas Carswell became the first UKIP MP, defecting from the Conservatives. A harbinger of the Brexit vote to come

Recent by-elections: 2024–2026

The period since the 2024 general election has produced several significant by-elections, all tracked on our by-elections tracker:

• Runcorn and Helsby (May 2025): Reform UK gained this Labour seat by a margin of just six votes after a recount, overturning a Labour majority of 14,696. A major shock result and Reform’s first by-election gain from Labour. Turnout was 46%. The result reflected Reform UK’s surge in the polls and Labour’s mid-term unpopularity
• Blyth Riverside (2026): Labour held this seat against a strong Reform UK challenge. The result demonstrated that Reform can push into second in northern seats but struggles to convert against well-organised incumbent Labour operations. See the full Blyth analysis
• Various 2025 contests: Labour faced significant swings against them in multiple seats, with Reform consistently pushing into second place in areas where Conservatives had previously been the main opposition

What by-elections tell us — and what they do not

By-election results are poor predictors of general election outcomes because of the structural differences above.

What by-elections can tell us:

• Direction of travel: very large swings against the government in multiple by-elections correlate with coming general election defeats
• Tactical voting patterns: which party opposition voters coalesce around in specific types of seat
• Issue salience: how strong an issue has to be to change vote choice
• Candidate quality effects: how much strong or weak candidates can move a result

What by-elections do not tell us:

• Whether a party will replicate their by-election swing nationally in a general election (they almost never do at full scale)
• Whether a party is on course for a particular seat count at the next election — use MRP models for this
• What will happen with normal general-election-level turnout

Reform UK in by-elections

Reform UK’s by-election trajectory is instructive. They have benefited from concentrated campaign efforts, high media coverage, and protest dynamics. Their vote shares in by-elections have consistently exceeded their national polling averages.

However, translating by-election performance into general election seats requires geographic concentration that Reform does not yet have across most of England. The Runcorn gain showed they can win in a specific set of circumstances; whether that is replicable across 50+ seats simultaneously at a general election is a different question. Our GE 2029 forecast addresses this.