Britain's Labour leader Ed Miliband on Wednesday accused the head of his party's biggest donor union of launching a "reprehensible" attack which risked tearing the opposition apart.
Len McCluskey, general secretary of the Unite union, told the New Statesman magazine that Labour would be "cast into the dustbin of history" if supporters of former prime minister and party reformer Tony Blair were allowed to set its course.
A spokesman for Miliband stressed that McCluskey did "not speak for the party.
"This attempt to divide the Labour Party is reprehensible," he added. "It is the kind of politics that lost Labour many elections in the 1980s.
"It won't work. It is wrong. It is disloyal to the party he claims to represent."
McCluskey named shadow foreign secretary Douglas Alexander, shadow defence secretary Jim Murphy and shadow work and pensions secretary Liam Byrne as weak links in the opposition cabinet.
"Ed Miliband must spend most of his waking hours grappling with what lies before him," he said.
"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.
"Liam Byrne certainly doesn't reflect the views of my members and of our union's policy," he added.
Murphy responded on Twitter, saying: "It's disappointing in advance of important local elections that Len McCluskey turns his fire on Labour."
Labour leader Miliband has fought off suggestions from the Conservative-led coalition government that he is beholden to the unions after they helped him secure the party leadership in 2010 at the expense of his brother David.
Blair last month warned the party that it risked becoming unelectable if it pitched too far to the left.