Britain's ruling Conservatives lost a parliamentary seat to the main Labour opposition Friday, as the country awaited local election results likely to pile more pressure on embattled leader Rishi Sunak.

Labour seized the constituency of Blackpool South, in the northwest of England, in the latest by-election defeat for the Tories as it appears on course to lose an upcoming general election.

The vote, triggered by a lobbying scandal that saw the area's Conservative MP resign, took place as voters cast ballots on Thursday in a mix of council, mayoral and other local contests across England.

Labour's Chris Webb won with a 26.3 percent swing -- the third largest margin from the Conservatives to Labour at a by-election since World War II.

"This seismic win in Blackpool South is the most important result today," said Labour leader Keir Starmer, tipped to be Britain's next prime minister.

The polls represent the last major ballot box test before Sunak goes to the country in a nationwide vote expected in the second half of the year.

His ruling Tories, in power nationally since 2010 and defending hundreds of seats secured the last time these local elections were held in 2021, are tipped to suffer heavy losses.

Early results showed that Labour was making gains in council seats, but all eyes were on key regional and London mayor races, the outcome of which are only expected later Friday and Saturday.

The capital's Labour mayor Sadiq Khan is expected to win a record third term easily, but mayoral contests in the West Midlands and Tees Valley, in northeast England, are predicted to be tight.

A victory for the Labour opposition in either of the regions, home to bellwether constituencies, would be hailed as further evidence voters are ready to return the party to power nationally.

Speculation is rife in the UK parliament at Westminster that a bad showing may lead some restive Tory lawmakers to try to replace Sunak, who has been in charge since October 2022.

Wins for the incumbent Tory mayors in the West Midlands and Tees Valley, Andy Street and Ben Houchen, would boost their hopes that the beleaguered leader can still revive their fortunes.

But with the Tories under fire nationally, on issues from water pollution to transport and inflation, Street and Houchen have appeared to distance themselves from the party during the campaign.

Pollsters forecast that the Conservatives could lose about half of the nearly 1,000 council seats they are defending in cities, towns and districts across England.

Worryingly for Sunak, the Conservatives only scraped into second place in Blackpool South ahead of the fringe Reform UK party, which threatens to squeeze the right-wing vote at the general election.