Labour needed a swing in the share of the vote of just 5.7 percentage points to win Blackpool South – but in the event they managed a huge 26.3 points.
It is the third largest swing from the Conservatives to Labour at a by-election since the Second World War.
The second largest swing took place earlier this year, at the Wellingborough by-election in February, which saw Labour take the seat from the Tories on a swing of 28.5 points.
The largest ever by-election swing from the Conservatives to Labour since the war was 29.1 percentage points, at the Dudley West by-election in December 1994.
Labour has now clocked up six by-election swings greater than 20 percentage points in the last 12 months.
Along with Blackpool South and Wellingborough, these are a 23.9-point swing at Tamworth and a 20.5-point swing at Mid-Bedfordshire, both in October 2023 and both from the Conservatives; 20.4 points at Rutherglen & Hamilton West in October 2023 from the SNP; and 23.7 points at Selby & Ainsty in July 2023 from the Tories.
The last time Labour recorded swings of this kind was when Tony Blair led the party in opposition from 1994 to 1997 – although the party only managed two such feats in this period: the 29.1-point swing at Dudley West in December 1994 and a 22.1-point swing at Staffordshire South East in April 1996.
The largest ever by-election swing against a government since the Second World War in a contest where a seat changed hands took place in July 1993, when the Liberal Democrats took Christchurch from the Conservatives on a swing of 35.4 percentage points.