The advantage recorded by Labour in the poll comes close to its best performance since Sir Keir Starmer became leader in 2019, and would put him on course for a comfortable overall majority in the House of Commons if repeated at the next general election.
It comes with the government reeling under the impact of the cost of living crisis, with energy bills forecast to soar to as much as £500 a month for some voters and the UK braced for a summer of strikes as workers demand pay rises to match expected inflation of 11 per cent.
Alarmingly for Liz Truss and Rishi Sunak, the candidates to replace Boris Johnson as Tory leader, the prime minister’s resignation seems to have accelerated the Tory plunge in the polls rather than arrested it.
Compared to a similar poll by Savanta a month ago, Tories shed five points, tumbling from 34 to 29 per cent, while Labour gained a point to move up from 41 to 42.
The previous poll was taken on the weekend of 25/26 June in the wake of the Partygate fines and just days ahead of the resignation of deputy chief whip Christopher Pincher in the sexual harassment scandal which was the final straw for Tory MPs who wanted Mr Johnson out.
This week’s results appear to reflect voters turning away from the party amid the vicious infighting of the battle to replace the PM.
And they suggest that events surrounding Mr Johnson’s removal have undermined support among the voters who backed Tories to a landslide victory under his leadership less than three years ago.
The proportion of 2019 Conservative voters who said they would back the party again in the next election plummeted from 80 to 70 per cent in the space of a month, as one in eight of their supporters abandoned them.
Of that group, one-fifth said they would switch to Labour, one-fifth to Lib Dems and two-fifths to Reform UK, the successors to the Brexit Party.
According to the Electoral Calculus prediction website, the figures recorded by Savanta could result in an overall Labour majority of around 70 seats, with Tories losing around 180 MPs to finish with a tally below 200 for the first time since 2005.
Among seats which could fall are Mr Johnson’s Uxbridge and South Ruislip, where the PM has a 7,210 majority over Labour, and deputy prime minister Dominic Raab’s Esher and Walton, which is one of the blue- wall constituencies under assault from Liberal Democrats.
Red-wall seats in the north and Midlands which would return to Labour after falling to Tories in 2019, on an even swing, could include Workington, Bolsover, Redcar and Bishop Auckland, as well as two seats each in Blackpool. Bolton, Bury and Leeds.
And other prominent Tories who could risk losing their seats could include Grant Shapps, Steve Baker, Anne Marie Trevelyan and even Jacob Rees-Mogg in North East Somerset.
The Lib Dems put on two points compared to last month to reach 12 per cent in the new poll.
Savanta questioned 2,272 British adults on 23-24 July and the results relate to those who said they were likely to vote in the next general election.