Jeremy Corbyn now has the support of more than half of those with a vote in the Labour leadership contest, a new opinion poll suggests.
Stark warnings from a string of senior party figures that choosing the veteran left-winger would be catastrophic for its chances of returning to power appeared to have had little effect.
The YouGov survey for The Times of 1,411 eligible voters in the contest to succeed Ed Miliband at the head of the opposition found Mr Corbyn had nearly doubled his lead in a week to 32%.
It gave him 53% - enough to win without a need to count second preferences - with Andy Burnham losing five points to 21%, Yvette Cooper slipping two to 18% and Liz Kendall down three on 8%.
In a finding certain to further fuel fears over the election process among Labour MPs and other critics, six in 10 of the surprise front-runner's new supporters are among newly-registered supporters.
The contest has been hit by claims that members of far-left groups and Conservatives are among around 190,000 to have signed up since the party's general election defeat in May.
They include affiliated supporters who - under new rules adopted under Ed Miliband - have to pay just £3 to take part as well as individual members of trade unions and other organisations which are affiliated to the party.
The latest signs of a serious challenge by Mr Corbyn came after Tony Blair's former spin doctor Alastair Campbell warned victory for the one-time rank outsider would be a "car crash".
In a strongly-worded intervention Mr Campbell called for Labour supporters to sign up to vote for "anyone but Corbyn", claiming that the leadership contest was now a battle to save the party.
He backed former Cabinet minister Alan Johnson in claiming that "the madness of flirting with the idea of Corbyn as leader has to stop".
Labour MP Barry Sheerman, a supporter of Ms Kendall, told the BBC the contest should be halted for an investigation into the "malign" motives of some of those signing up to vote.
Labour insists that it has "robust" systems in place to prevent "fraudulent or malicious" applications.
The findings were dismissed by the Cooper camp.
A spokeswoman said: "This does not reflect our extensive phone banking data, which does not suggest any single candidate will receive 50% of first preferences.
"Our figures - as other polls have suggested - show clearly that Yvette is the candidate best placed to draw support from all areas of the Labour Party and win this contest. She is best placed to unite the party and position Labour for victory in the 2020 election."
YouGov president Peter Kellner said he "would personally be astonished if Corbyn does not end up as Labour's leader" despite voting not starting until Friday and the result not being declared until September 12.
After adjustments were made to allow for possible factors that could skew the results of the polling, the outcome remained a first-round "knockout" victory for Mr Corbyn, he said.