Paul Kenny, the GMB's national secretary, told the union's annual congress in Brighton that he was serious about seeking a reversal on policies such as the backing for a freeze in public sector pay.
"We either get in the lift on the ground floor … or we are not getting in the lift at all. People need to understand that we are very serious about this," he said.
Kenny said the GMB would press Labour to sever ties with some of its New Labour-related groups, such as Progress. He warned that the public would not vote for a "Tory-lite approach from a Labour government".
However, Kenny ruled out a suspension of all Labour party funding, despite calls from the podium for a donation freeze. One motion called for a suspension until the shadow cabinet "fully commits to where possible righting the wrong and repairing the damage done by Tory-Lib Dem government".
The GMB's main executive body gave the motion qualified backing, saying a funding freeze was "too wide an action" and would hamper the union's ability to negotiate changes with Labour.
Another motion, which was not heard because of union rules, called for the GMB to de-affiliate from Labour. Raised by the Leeds civic branch, it said: "We cannot continue to waste money on them, we are simply funding the bullets they are using to shoot us with."
Ed Balls, the shadow chancellor, will address the congress on Monday afternoon and will take questions from the floor in what is likely to be an impassioned session.
Balls said that he backed George Osborne's imposition of public sector pay restraint until the end of parliament. Pay increases in 2013/14 and 2014/15 will be capped at 1%, after a freeze in the preceding two years.
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