The Labour leader has issued a stinging attack on the leader of the Unite union, a big Labour donor.
Unite general secretary Len McCluskey wrote an article for the New Statesman in which he singles out three senior members of the Shadow Cabinet - urging Ed Miliband not "to be seduced" by them - or risk losing the next election.
In response, a spokesman for the Labour leader described Mr McCluskey's comments as "reprehensible."
The Unite leader said: "Ed Miliband must spend most of his waking hours grappling with what lies before him.
"If he is brave enough to go for something radical, he’ll be the next prime minister. If he gets seduced by the Jim Murphys and the Douglas Alexanders, then the truth is that he’ll be defeated and he’ll be cast into the dustbin of history."
Of Shadow Work and Pensions Secretary Liam Byrne, Mr McCluskey wrote: “Liam Byrne certainly doesn't reflect the views of my members and of our union’s policy."
The Unite union is the largest single financial backer of the Labour Party.
In response, Ed Miliband's office said: "Len McCluskey does not speak for the Labour Party. This attempt to divide the Labour Party is reprehensible. It is the kind of politics that lost Labour many elections in the 1980s."
The Unite leader has also been leading the calls for a general strike, which was discussed at a meeting of union bosses at the TUC today.