Two polls spell bad news for Rishi Sunak, with one showing a drop of four points and the other that his party is on course to pick up just 72 seats.
A poll by Savanta for The Sunday Telegraph showed the Tories down four points to just 21% of the vote - the lowest by that pollster since the dying days of Theresa May's premiership in early 2019.
In a boost for Nigel Farage, the poll showed Reform UK up three points with 13% of the vote.
A separate Survation poll for Best for Britain, published by The Sunday Times, predicted the Tories would win just 72 seats in the next parliament, compared with 456 for Labour.
The result would give Labour a majority of 262 seats - far surpassing the landslide Labour achieved by Sir Tony Blair in 1997 - while the Liberal Democrats would pick up 56 seats, Reform seven and the Greens one seat.
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The Savanta poll, which was carried out from 12-14 June and involved 2,045 adults aged 18 and over, also showed Labour up two points on 46% of the vote.
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Chris Hopkins, political research director at Savanta, said the poll pointed to "nothing short of electoral extinction for the Conservative Party".
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"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," he said.
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"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."
The two surveys follow a YouGov poll on Thursday night that put Nigel Farage's party ahead of the Tories for the first time - on 19% of the vote, compared with 18% for the Conservatives.
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The development prompted Mr Farage to declare Reform as the "opposition to Labour" going into the election.
Mr Sunak has repeatedly argued that a vote for Reform would "give a blank cheque to Labour" - something Mr Farage has dismissed.
Survation surveyed 22,000 people and the poll was conducted between 31 May and 13 June. On 4 June, early in the polling period, Mr Farage announced he was the new leader of Reform and would be standing as a candidate in Clacton, Essex,
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The Survation results would mean that the Tories' vote share would have halved from 44% in 2019 to just 24%, while Labour would have increased theirs from 32% to 40%.
Speaking to journalists at the G7 summit in Italy on Friday, the prime minister said: "We are only halfway through this election, so I'm still fighting very hard for every vote.
"And what that poll shows is - the only poll that matters is the one on 4 July - but if that poll was replicated on 4 July, it would be handing Labour a blank cheque to tax everyone, tax their home, their pension, their car, their family, and I'll be fighting very hard to make sure that doesn't happen."
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Mr Sunak also claimed he would serve another five years in politics - wherever the outcome of the election.
Asked if he would commit to serving a full five-year term as prime minister if he wins the election, and to serving for five years as an MP if he loses, he replied simply: "Yes. Yes."
He also said "no" to a question about whether he planned to change campaign tactics in the light of the threat from Reform.