The broadside came from GMB leader Paul Kenny, who, speaking on behalf of 14 unions affiliated to Labour, said an interim document by former Labour General Secretary Lord Collins on the planned reforms - including changing the affiliation fee from automatic to voluntary - started, not ended, the process.
"The desire to expand party membership is a shared one but let nobody be under any illusion that as collective organisations, the removal or sale of our collective voice is not on the agenda."
He said the trade unions were not going to accept any advice on democracy and transparency from the "people who brought us the cash for honours scandals or whose activities are funded by cash from wealthy outsiders who refuse to give to the party but prefer to lay cuckoos in CLP nests".
Earlier, the Labour leader made clear there were no plans to change the power of the union block vote in this parliament; the focus was on the "huge change" to affiliation fees.
He also refused to issue an apology for the Falkirk selection fiasco, saying: "No. I don't think anybody can be proud of what happened in Falkirk" and that the party had recognised it needed to change how it worked and that it had "moved on".