The leader of Britain's largest public union has turned on Ed Miliband for his decision to endorse the coalition's public sector pay freeze.
Len McCluskey, general secretary of Unite, said the move was a "victory for discredited Blairism" and warned the Labour leader that he risked putting the party leadership on a collision course with its core supporters.
Officials from unions affiliated to Labour were left fuming over the weekend after Ed Balls told a conference that he backed the government's cap on the pay of millions of workers.
They privately warned that the move could lose Labour the next election, but McCluskey has used an article in the Guardian to voice their anger in public.
"Unions in the public sector are bound to unite to oppose the real pay cuts for public sector workers over the next year. When we do so, it seems we will now be fighting the Labour front bench as well as the government," he said.
"The political elite which was united in promoting the City-first deregulation policies that led to the crash is now united in asserting that ordinary people must pick up the tab for it.
"It leaves the country with something like a 'national government' consensus where, as in 1931, the leaders of the three big parties agree on a common agenda of austerity to get capitalism - be it 'good' or 'bad' - back on its feet.
He added: "Where does this leave the half a million people who joined the TUC's march for an alternative last year, and the half of the country at least who are against the cuts? Disenfranchised."
McCluskey strongly rejected the argument that pay restraint will help create jobs and he criticised the Labour Party for its failure to consult with trade unions before making the "shift" in policy.
The first trade union leaders knew of Balls' speech on Saturday was when they were contacted by the media for reaction.
McCluskey continued: "Notwithstanding that it impacts on millions of our members, it is hard to imagine the City being treated in such a cavalier way in relation to a change in banking policy.
"This confronts those of us who have supported Ed Miliband's bold attempt to move on from Blairism with a challenge. His leadership has been undermined as he is being dragged back into the swamp of bond market orthodoxy.
"Having won on the measures, 'new Labour' will likely come for the man sooner or later, and that way lies the destruction of the Labour Party as constituted, as well as certain general election defeat."
McCluskey also branded shadow ministers Liam Byrne, Jim Murphy, Stephen Twigg and Ed Balls as the "four horsemen of the austerity apocalypse" for endorsing "savage spending cuts".
Unite threw its weight behind Ed Miliband in the Labour leadership election and the unions now supply around 90% of the party's funds. This has lead accusations from some that Miliband is has to tow the union's line when setting out positions.
A spokesman for Miliband rejected McCluskey's attack. "Len McCluskey is wrong about our policy now and our approach for the future," the spokesman said.
"And Len McCluskey is wrong about what the last week showed for the Labour Party - a party united in it determination to pursue fairness even in tough times, make capitalism more responsible, and protect our priorities."
Labour deputy leader Harriet Harman said Labour's hand have been forced by the coalition's austerity measures: "When we left government the economy was beginning to grow and unemployment was beginning to fall but because they’ve squeezed the life out of the economy with their austerity cuts they are now in a position of having to continue with that public sector pay freeze. But what we don’t agree with is their proposals on regional pay bargaining, we don’t agree with their cuts in tax credit and we think they should make sure they do it as fairly as possible," she told BBC Radio 4's Today programme.