Ed Miliband's leadership lacks strategy, narrative and energy, and the Labour leader has "not broken through", according to his own adviser.
Labour peer Maurice Glasman, writing in the New Statesman, says Miliband has "flickered rather than shone, nudged not led."
In a further blow to the party, Glasman says that they have not won and "show no signs of winning" the economic argument.
"We have not articulated a constructive alternative capable of recognising our weaknesses in government and taking the argument to the coalition. We show no relish for reconfiguring the relationship between the state, the market and society."
Glasman also criticises the shadow cabinet, saying it is still dominated by "old faces from the Brown era".
The criticism comes after a difficult start to the year for the Labour leader.
A poll by the grassroots website Labourlist found the majority thought Miliband's performance in 2011 had been average or poor. Only a fifth believed Labour had had an excellent or good year.
It's not the first time Glasman has caused controversy. He once accused Labour of lying about immigration, and in an interview in July called for the renegotiation of the party's relationship with the EU to restrict migrants.
Although internal Labour mutterings about Ed Miliband have grown markedly in the past couple of months, particularly since a fairly disastrous PMQs performance just before parliament broke up for Christmas, some pundits who don't want to see a Labour government are urging the plotters to keep quiet.
The Guido Fawkes blog has in recent days revived a campaign called the Don't Unseat Ed Miliband Association (DUEMA), a group of Tory supporters who think the current Labour leader is a sure-fire guarantee of the party staying in opposition.