Jeremy Corbyn is now the "red-hot favourite" for the Labour leadership, with bookmakers slashing their odds after an opinion poll indicated he was on course for victory.
The backbench MP now has the support of more than half of those with a vote in the Labour leadership contest, the poll suggested.
The Corbyn campaign has continued to gather momentum despite warnings from a string of senior party figures that choosing the veteran left-winger would be catastrophic for Labour's electoral chances, with one grandee comparing him to former Tory leader Iain Duncan Smith.
The YouGov survey for The Times of 1,411 eligible voters in the contest to succeed Ed Miliband 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%.
William Hill cut Mr Corbyn's odds twice in just two hours as money piled in for the left-winger following the survey, with bets of up to £2,000 recorded.
At 9am he was a 5/4 shot, yet by 10am he had been trimmed to 1/2 and less than an hour later he was as short as 1/3.
Spokesman Graham Sharpe said: "We can recall no other example of a 200/1 chance becoming an odds-on favourite in a political betting market in our 50-plus year history of political betting."
Ladbrokes said Mr Corbyn was the "red-hot favourite" after his odds collapsed in recent weeks.
The prospect of a win for the Islington North MP has rocked the Labour establishment, with figures on the party's right warning that he would ruin any chance of forming the next government.
Tony Blair's former spin doctor Alastair Campbell has urged Labour supporters to sign up to vote for "anyone but Corbyn" to help the party "stop itself driving over a cliff".
Lord Soley, a former chairman of the Parliamentary Labour Party, compared the prospect of a victory for Mr Corbyn to Mr Duncan Smith's disastrous leadership of the Conservatives.
Serial rebel Mr Corbyn had been accepted as a "maverick" within the party but discipline was needed in parliament, Lord Soley said.
He added: "In his many years as an MP Jeremy put his individual judgment before the collective judgment of the parliamentary party.
"As a party we accepted that because we always accepted that there is a place for mavericks, but if the maverick becomes the leader who called for individual judgment to be used over and above collective decision-making, then the message is clear.
"The party in Parliament becomes a collection of individuals - not a political party. That might be exciting in an party election campaign but it is a joy for the Tory party as it cements their position in power.
"It might focus minds before this important vote if we recall how delighted we were when Iain Duncan Smith became leader of the Tory party. We wanted him to stay. We should not fall into the same careless way of thinking."
Ms Cooper stressed the party had to be able to win a general election to put its principles into practice.
She told Sky News: "There is a lot at stake in this leadership election. This is about whether we can be true to our values but also strong enough to win."
Speaking from a children's centre, she said: "The future for the kids at this Sure Start centre depends on whether Labour can get back into government, otherwise they will be hit with closure and those children here now will be growing up, their entire childhood, under a Tory government."
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.