The United Kingdom's opposition Labour Party inflicted a series of defeats on the governing Conservatives, gaining control of a string of councils and the only constituency seat that was up for grabs.
The losses could prove embarrassing for British Prime Minister Rishi Sunak, and difficult for him to ride out ahead of an upcoming general election.
How bad are the results for Sunak?
Labour took the constituency of Blackpool South in the northwest of England. The electoral district is part of the so-called "Red Wall" of normally Labour seats that former Prime Minister Boris Johnson flipped to the Conservatives in the last election in 2019.
Labour's Chris Webb won the seat with a 26.3% swing away from Sunak's Tories — the third largest by-election shift from the Conservatives to Labour since World War II.
Meanwhile, early results on Friday showed Labour making significant gains in local council seats. All eyes are also on key regional and London mayor races, for which the outcomes are only expected later on Friday and Saturday.
The Blackpool South defeat was the Conservatives' 11th by-election loss since Johnson won his 2019 landslide victory. Sunak, whose predecessor Liz Truss was forced to quit after just 49 days in office, has been at the helm for seven of those losses.
Worryingly for Sunak, the Conservatives only just secured second place in the Blackpool South election. The fringe far-right Reform UK party, which threatens to split the right-wing vote for Sunak's Tories, won 17% of the vote. That is Reform's best-ever by-election performance.
The local elections only took place in England and Wales, with no voting in Scotland or Northern Ireland.
How good are the results for Labour?
For the opposition center-left Labour Party, the results appeared encouraging with the party winning control of some councils it hasn't led for decades.
Labour leader Keir Starmer said the latest by-election victory had delivered an important message.
"This seismic win in Blackpool South is the most important result today," Starmer said. "This is the one contest where voters had the chance to send a message to Rishi Sunak's Conservatives directly, and that message is an overwhelming vote for change."
London's Labour Mayor Sadiq Khan was expected to win a record third term easily, but the party also hopes to make inroads in mayoral contests in the West Midlands and the Tees Valley. Both are currently Conservative-held and results there are predicted to be tight.
Victory in either of those areas, both including some bellwether constituencies, would be seen as further evidence that Starmer will be elected UK prime minister at a general election expected later this year.
However, it wasn't all good news for Labour with poor results in some areas with large Muslim populations, such as Oldham in northwest England.
In those places, the party's candidates appear to have suffered as a result of Starmer's perceived pro-Israel stance in the conflict in the Gaza Strip.
