Britain's ruling Conservatives on Saturday braced for further losses in local elections, their worst results in recent memory and a key test before a general election to be held by January.

Prime Minister Rishi Sunak's party lost nearly 500 councillors and control of 12 councils as well as a parliamentary seat, after voting on Thursday across a swathe of England saw the Labour opposition make huge gains.

The beleaguered Tory leader faces more likely defeats when results of crunch mayoral races -- in London, Liverpool and Manchester -- are announced later, but is hoping to retain the West Midlands.

In a rare success on Friday, a Tory mayor won a third term in Tees Valley, northeast England -- albeit with a vastly reduced majority.

Labour, out of power since 2010 and trounced by Boris Johnson's Conservatives at the last general election in 2019, seized on its claim to the Blackpool South seat in parliament and other successes to demand a national vote.

"Let's turn the page on decline," Labour leader Keir Starmer told supporters on Saturday in the East Midlands, where the party won the mayoral race.

Sunak must order a general election be held by January 28 next year at the latest, and has said he is planning on a poll in the second half of 2024.

Writing in Saturday's Daily Telegraph, Sunak conceded the returns showed "voters are frustrated" but insisted "Labour is not winning in places they admit they need for a majority".

"We Conservatives have everything to fight for," Sunak concluded.

Labour has enjoyed double-digit poll leads for all of Sunak's 18 months in charge, as previous scandals, a cost-of-living crisis and various other issues dent the Tories' standing.

On Thursday, they were defending nearly 1,000 council seats, many secured in 2021 when they led nationwide polls before the implosion of Johnson's premiership and his successor Liz Truss's disastrous 49-day tenure.

With almost all those results in by Saturday morning, they had lost close to half.

If replicated in a nationwide contest, the initial tallies suggested Labour would win 34 percent of the vote, with the Tories trailing by nine points, according to the BBC.

Sky News' projection for a general election using the results predicted Labour will be the largest party but short of an overall majority.

Its by-election scalp in Blackpool -- on a mammoth 26-percent swing -- was the Conservatives' 11th such loss in this parliament, the most by any government since the late 1960s. (AFP)