Theresa May's shattered authority suffered a fresh blow as a major study showed Jeremy Corbyn heading for No 10.
The Conservatives would lose 59 seats at a general election, making Labour the largest party in the Commons.
Labour would gain 34 seats and the Lib Dems 14 according to the analysis from Electoral Calculus for the Sunday Telegraph.
Experts blamed the shock plunge in support on Tory voters' anger "at the Government's failure to deliver Brexit ".
British Polling Council, President Professor Sir John Curtice said Leave backers had been "drawn back to either UKIP or Nigel Farage's newly launched Brexit party".
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Martin Baxter, the Electoral Calculus founder, told the Telegraph: “ Theresa May is discovering why David Cameron really held the referendum.
"It wasn’t to placate his own Eurosceptic MPs, instead it was to stop Conservative voters defecting to pro-Brexit parties.
"That process seems to have restarted and the Conservatives are beginning to suffer.”
It came as Tory big beasts said party rules could be changed – making it easier to oust the Prime Minister. Under the current system, a move against the leader can only be brought once in a 12-month period.
Mrs May rebuffed a bid to oust her last December and so would not expect to face the risk of another possible attempt to topple her until the end of the year.
However, two former chairmen of the 1922 Committee of backbench Tory MPs said rules could be changed.
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Lord Spicer and Lord Hamilton said the 12-month rule on no-confidence votes had been "interpreted as being immovable". "
They wrote: "It is reported that in order to change this rule the whole Conservative Party constitution must be opened, a National Convention called and even that a petition of 10,000 members is required.
"This is not the case," they said.
