Len McCluskey, the general secretary of the Unite union, has attacked Labour MPs on the right of the party for pandering to the Daily Mail and "right-wing media".
McCluskey has been engaged in a prolonged row with Ed Miliband over the influence of unions over the party. Miliband wants union members to opt into party membership rather than be automatically affiliated.
Speaking at the Labour Party conference in Brighton on Monday morning, McCluskey lashed out at Labour MPs who "shudder when trade unions are mentioned, fearful of a bad headline in the Daily Mail".
He added to applause: "I say to them, you will never ever appease the right-wing media and to try demeans you and our party."
Miliband, who had been sat on the stage before McCluskey rose to spoke, had vanished. "I'm sorry Ed's left the platform," the union leader joked.
The union leader received a standing ovation from the audience in the Brighton conference centre - much to the delight of the Conservative Party.
The Tory press office Tweeted: "Len McCluskey gets a standing ovation at Labour conference. His grip over Labour and weak @Ed_Miliband is as strong as ever #SameOldLabour."
The Labour leader's plans to reform the union link with the party has proved highly controversial, and led the GMB union to decide to cut its affiliation funds to the party by over £1m from next year.
GMB general secretary Paul Kenny told the conference last night: "We are certainly not going to accept any advice on democracy and transparency from the people who brought us the cash for honours scandals."
And last night McCluskey said Labour could not abandon its roots. "My challenge to Ed Miliband is to make it clear that the Labour Party is the party of working people, is the party of organised labour," he said. "Labour has no god-given right to exist – it can only exist if it speaks for ordinary working people. Demonstrate that you’re on our side."
Union leaders are also due to demand that Labour no longer opposes scrapping the public sector pay freeze. In a motion to be heard by delegates at the party's conference in Brighton, Unison says the Labour leadership must end its opposition to lifting the cap on public sector pay.
Until now, the Labour leadership has insisted they would not lift the the cap - imposed by the coalition Government - as part of efforts to reduce public sector spending.
The position has angered unions and today Unison has called for a debate, which is expected to be heard at about 3pm in the main hall. Dave Prentis, general secretary of Unison, will speak during the debate.