Labour is on course for a 262-seat majority, analysis by pollster Survation has suggested.
The analysis and modelling based on more than 40,000 surveys indicates Labour is ahead in 456 seats, with the Tories in first place in just 72.
Meanwhile, a voting intention poll by Savanta also contained bad news for Rishi Sunak, with a warning the Tories could face “electoral extinction”.
The Survation model puts the Liberal Democrats on 56 seats, the SNP on 37, with Reform UK currently favourites in seven seats.
Nigel Farage said the poll showed “the momentum is building” for Reform.
The analysis suggests Plaid Cymru are on track for two seats and the Greens would hold Brighton Pavilion.
The Survation study for campaign group Best For Britain used the multilevel with poststratification (MRP) technique to model results in constituencies.
Survation polled 42,269 people online or over the telephone between May 31 and June 13.
It is the first MRP analysis since Mr Farage returned to the political frontline.
In its interpretation of the findings, Survation said: “Since Farage’s announcement to take over as leader of Reform UK, we’ve seen a rise in their vote share in national polling, and now we are seeing how this can result in seat gains.
“Unsurprisingly, Reform are making significant gains in places where the Conservatives are losing the most, and are currently the leading party by vote share in seven seats. Reform are also currently performing better than the Conservatives in 59 seats.”
The Savanta study for the Sunday Telegraph gave Labour a 25-point lead, with Sir Keir Starmer’s party on 46 per cent, up two from last week, and the Tories on 21 per cent, down four points.
It is the lowest share that the Conservatives have had with the pollster under Mr Sunak.
Chris Hopkins, political research director at Savanta, said: “Our research suggests that this election could be nothing short of electoral extinction for the Conservative Party.
“The hopes of Conservative candidates are being shot to pieces by poll after poll showing the Conservative Party in increasingly dire straits – and we’re only halfway through the campaign.
“There’s a real sense that things could still get worse for the Conservatives, and with postal votes about to drop through millions of letterboxes, time is already close to running out for Rishi Sunak.”
Reform UK were on 13 per cent, up three points, the Liberal Democrats up two points on 11 per cent, the Greens up one point on 5 per cent and the SNP down one on 2 per cent.
Savanta surveyed 2,045 UK adults from June 12-14.
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