Unite general secretary Len McCluskey has issued a warning to Ed Miliband that Labour can not be a "pinkish shadow" of the coalition if it is to energise union members to join the party.
Speaking to a meeting of Unite activists on Wednesday, he said strains in the link between Labour and the unions have been building up for years, at least since the advent of New Labour in the 1990s.
"Those strains have been fuelled by the failures and disappointments of Labour in office, not least its refusal to seriously address the unique legal impediments trade unions have to work under in Britain.
"They have been worsened by the feeling that for a long time we have been taken for granted by people who welcome our money, but not our policy input, who want to use our resources at election time but do not want our members as candidates.
McCluskey attacked the "Oxbridge Blairites" within Labour that he said wanted to limit union influence as they saw parliamentary candidate selection as their "feudal right"
"Tory media and New Labour spin doctors will never be able to understand the solidarity of working people," he said.
McCluskey welcomed changes to the historic arrangements for affiliating union members to Labour which have been put forward by Miliband. A special conference will be held next Spring to agree details of the changes, under which union members would have to agree to opt-in to Labour membership.
But he suggested support for reforms was dependent on Miliband leading a "Labour party that our members want to support, because they believe it can and will make a difference in their lives" and "not a party that is a pinkish shadow of the present coalition that gives the City a veto over economic decisions and embraces the austerity agenda squeezing the life out of the country."
The proposed changes followed accusations that the union had stitched-up the selection of the Labour candidate in Falkirk. David Cameron and Conservative MPs have put Miliband under sustained pressure in recent weeks over Unite's influence over the party.
However in a robust defence of his union's activities, McCluskey said it "did nothing wrong". He said the party's response, which included suspending two people from the party, was down to Labour figures in Westminster having a "default setting of panic" and who were "enslaved to the media agenda."
He savaged a Labour Party report into the affair as a "shoddy farce" produced to "trample on the rights of the Falkirk constituency to smear and slander our union".
"We'll never accept that what has been done by the Labour Party in Falkirk is right, we will ensure that the truth will come out," he said.
And he said the decision to involve the police in the matter was an attempt to sow fear within the union. "The police should be looking after rapists and murderers rather than wasting their time with this," he said.
The union leader saved some of his harshest words for journalists - in particular the Daily Mail. "We know you are sleazy bullies, we know you like to kick the weak while licking the boots of the powerful," he said.