The attack came on the same day a YouGov poll showed Miliband is seen as less trustworthy, decisive and competent than his much-scorned predecessor, Gordon Brown.
In an interview with LondonlovesBusiness.com, JML chairman John Mills, said Labour lacks a "clear idea on how to get the economy growing again".
Mills claimed Miliband and shadow chancellor Ed Balls had failed to convey a "credible" economic message to the public and had not shown they had a "clear idea on how to get the economy growing again at a reasonable speed and how to get the deficit down to a much more sustainable proportion".
“The problem is that anybody who is in opposition always looks like a pygmy - this is just a fact of life. Anybody who is in power looks much bigger and more powerful than people want,” he said.
Mills is the largest donor to the Labour Party this year giving twice as much as the Unite union.
In a poll published in The Times today, only 19% said Miliband was decisive or strong. A similar poll last year found only 20 per cent thought he was PM material.