Jeremy Corbyn's "starry-eyed hard-left" economic strategy would push up inflation and interest rates, undermine support for public services and deliver a decade or more of Conservative rule, shadow chancellor Chris Leslie has warned.
Mr Leslie, who is backing Yvette Cooper for the Labour leadership, delivered his scathing assessment following polls putting the left-wing candidate at the head of the race to succeed Ed Miliband.
Meanwhile, Liz Kendall - widely seen as the most Blairite of the four contenders - took aim at Mr Corbyn for promoting an agenda which was "neither realistic nor electable" and said that she would offer a"hopeful alternative" for Britain.
In a clear attempt to broaden her appeal beyond the centre and shake off leftists' accusations that she is a "Tory", Ms Kendall set out five causes which would be at the heart of Labour's agenda if she wins - ending inequality from birth, eliminating low pay, building a caring society, sharing power and offering a future of hope for young people.
A report in the Sunday Times named Mr Leslie, Ms Cooper and Ms Kendall among eight eight senior figures currently attending shadow cabinet who would refuse to serve under Mr Corbyn if he is named leader on September 12. One unnamed senior party figure told the paper Mr Corbyn would struggle to field a complete team of shadow ministers.
But the veteran backbencher indicated he would seek to build a shadow cabinet drawn from all wings of the Labour Party if elected, saying: "I have to be big enough to accommodate those differences of opinion and I understand that."
Mr Leslie told The Independent Mr Corbyn's plans to fund infrastructure investment by printing money would "push up inflation, lending rates, squeeze out money for schools and hospitals and mean spending more on debt servicing", with the worst impact hitting those unable to pay privately for services.
Mr Corbyn would be unable to eliminate the deficit through tackling tax avoidance and cutting tax reliefs, leading to further cutbacks in public services, with the result that "taxpayers will lose faith in the public realm, will become more sceptical and cynical, and be more likely to exit and go to private health and private education", he predicted.
Mr Leslie said: "You have got to have a credible Labour prime minister who understands this. Otherwise you have a decade or more of Tory rule."
Ms Kendall acknowledged the appeal of Mr Corbyn's anti-austerity platform, telling The Independent: "After a traumatic defeat, it's understandable that people want to reach out for a radical option that appears to offer hope for party and country alike."
But she insisted that the veteran left-winger "doesn't have a monopoly on hope", and said his "solutions based on conventions of the past" will not help Labour "win the national conversation in the years ahead".
The Leicester West MP accepted that Labour "lacks the public's trust" and warned: "Unless we can restore that, we won't deliver on the causes we believe in."
She said: "We must offer a realistic prospect of that new nation we wish to build, not the false hope of policies that are neither realistic nor electable. The scale of the challenge we face demands boldness and radicalism. A timid offer to the British people isn't the alternative."
Ms Kendall promised to restore working tax credits and maintenance grants for poorer students, and said she would spend an additional £1 billion on pre-school education, funded by reversing Chancellor George Osborne's inheritance tax cut from the summer Budget.
She said she would reverse Conservative legislation restricting unions' ability to call strikes, introduce online ballots for industrial action, guarantee rises in public sector wages and end the "scandal" of low pay for carers.
Ms Kendall's five causes will form the basis of an online advertising campaign to run over the fortnight before members and registered supporters receive their ballot papers.