A leading centre for independent economic research has said the best evidence currently available suggests a 50p tax rate backed by Labourwill raise "little revenue" and make a "marginal contribution" to reducing the budget deficit.
The Institute of Directors (IoD) said Labour must "drop its practice of knee-jerk reversion to the old socialist nostrums" if the party wants to be taken seriously by business.
Director general Simon Walker said: "The 50p tax rate - actually 52p, because the last Labour government manipulated national insurance contributions - greatly damaged Britain's claim to being seen as a low-tax economy and actually drove down total tax receipts.
"It was, and remains, an envy-driven political gesture designed solely to drive a wedge between voters."
He added, "It will significantly damage Labour's credibility with the business community."
Ed Miliband answered questions for the first time this afternoon about his plans to re-introduce the 50p rate of income tax.
Politicians at both ends of the political spectrum need to be more honest about the true motivations behind their tax policies.
Ed Balls has said Labour would restore the 50p tax rate for those earning over £150,000 if the party wins the next election.