The lead was a record high share for Labour in any YouGov poll as well as the highest figure the party has ever recorded in any published survey since the late 1990s, YouGov said.

Earlier on Thursday, British Prime Minister Liz Truss defended her controversial plan to reignite economic growth after huge tax cuts unveiled last week hammered the value of the pound and government bond prices.

The YouGov poll of voting intention conducted over Wednesday and Thursday showed 54% support for Labour and 21% for the Conservatives. It was a survey of more than 1,700 British adults.

Another YouGov poll earlier this week had shown 45% of voters backing Labour compared to 28% support for the Conservatives.

Truss took office on Sept. 6 after defeating former finance minister Rishi Sunak to win the Conservative Party's leadership contest. The next national election is likely to be held in 2024.

At least two Conservative lawmakers - both of whom had backed Sunak during the party leadership race - publicly criticised Truss's economic plans following the release of the poll.

"This is now a serious crisis with a lot at stake," Conservative lawmaker George Freeman said on Twitter. "The economic package of borrowing & tax cuts announced last week clearly can't command market or voter confidence."

Another lawmaker from Truss's party, Julian Smith, urged the government to reverse the abolition of the 45% top rate of income tax.

Three other polls on Thursday also showed large leads for Labour - Survation put Labour's lead over the Conservatives at 21 points; Deltapoll showed Labour 19 points ahead; and Redfield & Wilton Strategies had Labour 17 points ahead.

Opposition leader Keir Starmer said during his party's annual conference this week that it was Labour's best chance to win power since 2010, following four straight election defeats.