Jeremy Corbyn has been accused of plotting to take Labour back to the failed policies of the 1970s after he suggested he could seek to restore the party's historic commitment to the public ownership of industry.
Leadership rivals Yvette Cooper and Liz Kendall said the veteran left-winger was trying to "turn the clock back" after he hinted he could re-instate Clause IV, which enshrined "common ownership of the means of production" in the party's constitution.
His campaign team later issued a "clarification" insisting he was not advocating a return to Clause IV- famously scrapped by Tony Blair 20 years ago in one of the defining moments of his leadership.
However, a spokesman confirmed that Mr Corbyn would seek to open up a discussion about "public ownership objectives for the 21st century" - including the railways - if he won the race to succeed Ed Miliband.
His words were seized on by Ms Cooper who said that Labour did not need a return to "the days of British Leyland" - the nationalised car manufacturer which became a byword for shoddy products and industrial strife.
"Labour needs radical ideas for the future, not to turn the clock back. We've always been a progressive party that's embraced the future - this is not the time to be reactionary and cling to the past," she said.
Ms Kendall - the leadership challenger seen as being the closest to Mr Blair's policies - said Mr Corbyn represented a throwback to the failed ideas of left-wingers like the late Tony Benn.
"This shows there is nothing new about Jeremy Corbyn's politics. It is just Bennism reheated, a throwback to the past, not the change we need for our party or our country," she said.
"Life had moved on from the old Clause IV in 1994 let alone 2015. We are a party of the future, not a preservation society."
The row came amid fresh warnings from wealthy Labour donors that they would stop giving to the party if Mr Corbyn becomes leader.
Assem Allam, the multi-millionaire owner of Hull City, told The Sunday Telegraph that he did not want a leader "controlled by trade unions" while businessman Richard Brindle said Mr Corbyn's policies were "economically illiterate".
The increasingly strident warnings about the dangers of a Corbyn victory appeared to reflect a deepening concern among the established Labour leadership that his campaign could be developing an unstoppable momentum.
In an interview with The Independent on Sunday, the Islington North MP said he wanted to develop a "clearer set of objectives" in relation to state ownership.
"I think we should talk about what the objectives of the party are, whether that's restoring the Clause IV as it was originally written or it's a different one, but I think we shouldn't shy away from public participation, public investment in industry and public control of the railways," he said.
Any move to restore Clause IV would be every bit a symbolic as Mr Blair's original "Clause IV moment", which was widely seen as crucial in helping to convince voters Labour could be trusted in government after the wilderness years of the 1980s.
A spokesman for Mr Corbyn said: "Jeremy is not saying he wants to return to Clause IV, nor does he want a big 'moment' such as that. He says we need some forms of discussion about public ownership objectives for the 21st century."