United Kingdom by-election records

UK by-election records is an annotated list of notable records from UK Parliamentary by-elections. A by-election occurs when a Member of Parliament resigns, dies, or is disqualified or expelled, and an election is held to fill the vacant seat. A constituency is the seat or division that member represented.

Prior to 1945, electoral competition in the United Kingdom exhibited features that make meaningful comparisons with modern results difficult.

Among the most significant were:

Since 1945, the evolution of a stable three-party system has tended to negate each of the above features so that, broadly speaking, elections are more comparable.

In Northern Ireland, as ever, the pattern of party competition is completely different from that on the mainland and comparisons remain problematic.

Hence, unless otherwise stated records are based on results since the 1945 General Election, and earlier exceptional results are listed separately.

For comparison purposes the following definitions have been adopted.

For more information about what is meant by the term "swing", see Swing (politics)

¹ By-elections where the seat was held by the incumbent MP.
² By-elections where the seat was held by the incumbent party.
³ By-elections contested by the incumbent MP, who failed to gain re-election.

A party's share of the vote at a general election is not always matched at subsequent by-elections, but given the five-year maximum term of a Parliament, reductions of 20% or more are unusual. Those of 25% or more are listed below:

Worst results for other parties:

Winning shares of the vote below 35%, since 1918:

The , was held to elect two MPs. The winners' shares of the total vote were 25.6% and 25.1%. However, as each voter could cast two votes, the situation is not readily comparable to other by-elections in this period.

At the Sheffield Attercliffe by-election, 1909, the winning candidate took only 27.5% of the vote.

Major parties winning 2% or less share of votes cast in a by-election, since 1918:

The worst Conservative performance was in the North Down by-election, 1995, where they took 2.1% of the votes cast. The 'continuation' Social Democratic Party (SDP) took 0.4% of the vote at both the Upper Bann by-election, 1990 and the Bootle by-election the following week.

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