Simon Wren-Lewis warns of a "terrifying" drop into electoral irrelevance.
Two of Jeremy Corbyn's economic advisers have backed his challenger, Owen Smith, after concluding the embattled leader cannot provide proper opposition.
Simon Wren-Lewis, an Oxford University professor, and David Blanchflower, a former Bank of England Monetary Policy Committee member, have made their reservations public.
Wren-Lewis was responding to Corbyn supporters who claimed they were rebuilding "a mass social democratic party"on his blog.
He said: "My response is the same as any decent social scientist: show me the evidence that this is what you are doing."
He praised Corbyn's ability to mobilise an activist base of young idealists, but added: "What terrifies MPs, and me, is if this base gets delusions of idealism and grandeur, and saddles them with a leader who will lead the party into electoral irrevelance.
"If you think those fears are wrong, show me your evidence."
Later, responding to comments, he said: "I want Smith to win."
Blanchflower tweeted a link to the article and declared: "Member of econ advisory group joins me in supporting [Owen Smith]." Damningly, he told The Guardian that Smith had been better at consulting businesses and economists in three weeks than Corbyn had in nine months.
Both Blanchflower and Wren-Lewis were appointed to Labour's Economic Advisory Committee, which was created by the shadow Chancellor John McDonnell.
Their declarations come after a poll showed the Conservatives enjoying a 16-point lead over Labour.