The big boss of Unite, Len McCluskey, used an article in The Guardian this morning to attack Ed Balls' and Ed Miliband's direction on austerity measures.
McCluskey criticised the party's change in direction as a "Blairite policy coup".
“Liam Byrne, Jim Murphy, Stephen Twigg and now Ed Balls: four horsemen of the austerity apocalypse," he wrote.
"Ed Balls's sudden embrace of austerity and the public-sector pay squeeze represents a victory for discredited Blairism at the expense of the party's core supporters. It also challenges the whole course Ed Miliband has set for the party, and perhaps his leadership itself.
"No effort was made by Labour to consult with trade unions before making the shift, notwithstanding that it impacts on millions of our members," McCluskey continued. "It is hard to imagine the City being treated in such a cavalier way in relation to a change in banking policy."
It's very different to the line he gave Total Politics in an interview last March.
Defending both Eds and the Labour Party's new direction, McCluskey told me:
"Ed Miliband was only elected six months ago. It takes a while to get your feet under the table. He has to deal with the internal problems: a number of ex-Blairites within the party who didn’t want him elected as a leader still have a powerful voice. But I think that’s starting to coming together. Ed Balls coming in as shadow chancellor has strengthened the team and we’re starting to see a more aggressive approach. "
He also praised Ed's approach to the Labour policy review.
"I think Ed’s approach to this – a blank sheet of paper – was absolutely the right way to go about it. He got attacked and vilified by some in the right-wing media and some in the party but he should have been proud of making that statement. It’s not a blank sheet of paper.
"It’s within the confines of our believes and values but what he was saying is it’s time to draw a line, the policies of New Labour, the adherence to the market was wrong. The abandonment of any regulation of our financial institutions was wrong and we should start again in many ways and see where the disconnect came from and start from there. It was a brave thing to do and we applaud that and we will be involved in that enthusiastically."
Trade union Unite is the biggest donor to the Labour Party.
But it's not the first time a Labour Party-supporting trade union has hinted at their discontent with Ed Miliband.
Paul Kenny, the general secretary of the GMB, told me in an interview at Labour Party conference last year: "I think he [Ed Miliband] should just be himself. Look, I can’t foresee a day when the Daily Mail or The Telegraph – or even The Sun at this stage anyway - are going to put across their front page, ‘Ed: what a stunner.’ It just ain’t going to happen. We’ve got to accept that."
While some will dismiss McCluskey's intervention as deficit denying or a cry for attention, the more personal attacks, like the one from Paul Kenny, could do more damage to Ed Miliband's reputation.
It'll be interesting to see how Ed's team responds.
UPDATE: Miliband has just released the following statement...
“Len McCluskey is entitled to his views but he is wrong. I am changing the Labour Party so we can deliver fairness even when there is less money around and that requires tough decisions.
“It requires a tough decision to put the priority on jobs ahead of public sector pay.
“It also requires us to say we do believe the government is going too far, too fast with their cuts but we are not going to make specific promises to reverse those cuts unless we are absolutely sure that we know where the money is coming from.
“That is right, it is responsible and it is the way we are going to proceed.”