Labour leadership candidate Yvette Cooper has issued a call for party members to "get serious", after a private poll suggested radical left-winger Jeremy Corbyn has stormed into a 20-point lead.
Ms Cooper warned that the party would be "condemning our world to a Tory future" if Mr Corbyn wins the race to succeed Ed Miliband.
The backbencher is poised to top the ballot with 42%, with Ms Cooper on 22.6%, Andy Burnham on 20% and Liz Kendall on 14%, according to the study seen by the Daily Mirror.
Mr Corbyn's lead narrows once second preferences have been taken into account, putting him just two points ahead on 51% while Ms Cooper comes in second on 49%.
It follows a public opinion poll last week that put the Islington North MP in line for a shock victory.
Ms Cooper warned that Labour members face "a choice of two futures".
"I'm setting out a Labour alternative to the Tories in a host of areas because I want Labour to be able to change the world once again," she told The Independent.
"But we can only change it if we win. If we make the wrong choice in this election, we will be condemning our world to a Tory future."
Ms Cooper said the surge in support for Mr Corbyn's anti-austerity message was driven by "frustration and anger" at the prospect of five more years of Conservative government.
But she warned that Labour must not "go back to the 1980s and just be a protest movement", arguing that the party can only change the world by defeating the Conservatives in the 2020 election.
Mr Corbyn moved to clarify his stance on Europe after he was the only one of the four leadership candidates not to make an emphatic commitment to campaigning in favour of continued EU membership in the referendum planned by the end of 2017.
In a statement to The Guardian, he said: "Labour should set out its own clear position to influence negotiations, working with our European allies to set out a reform agenda to benefit ordinary Europeans across the continent.
"We cannot be content with the state of the EU as it stands. But that does not mean walking away, but staying to fight together for a better Europe."
A spokesman for Ms Kendall dismissed the findings of the reported poll, saying: "A poll is something done by a recognised company with data published for all to see. This isn't a poll, it's just spin, and rather desperate stuff at that."