Trade unions will only back Ed Miliband's union funding reforms if Labour stops being a "pinkish shadow of the present coalition", Len McCluskey has said.
The Unite general secretary said he hoped to support Miliband's reforms because they offered the possibility of "enhanced policy input" for unions.
But he made clear support for the changes would come at the price of increased union influence on Labour policy - to the extent that the party abandons its support of the bulk of the coalition's deficit reduction measures.
"Our main aim is to ensure that as many Unite members as possible, already paying our political levy, now sign up individually, by whatever means have transparency and integrity, to be affiliate members of the party," McCluskey told a meeting of Unite representatives and activists today.
"For that to work, and for the trade unions to put their shoulders to the wheel to make it work, the offer has to be an attractive one."
Miliband is proposing changing the way in which unions fund the Labour party, by making the party affiliation an explicit individual one.
The Labour leader has called a special conference taking place next spring, which he hopes will resolve the issue, but support from Unite now seems conditional on Miliband lurching to the left.
"Above all, [support for the reforms] means a Labour party that our members want to support, because they believe it can and will make a difference in their lives," McCluskey added.
"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.
"But a party that offers real hope, that stands up for the poor and vulnerable, that puts growth at the heart of its agenda, that confronts privilege."
He demanded union party members should be given the right to choose Labour candidates in constituencies, have an "equal say" in the election of the party leader and the "right to a level playing field in the party where their views and votes count as much" as other party members.
Unite was the union which triggered Miliband to shake up the historic relationship with the Labour party, after allegations that the union was attempting to influence the selection of Labour candidates in 40 constituencies.
The changes are expected to result in an overall reduction in funding for the Labour party. Polling by Tory peer Michael Ashcroft yesterday showed just 12% of Unite members would choose to affiliate with Labour.
McCluskey warned Unite would use its political fund for a range of activities, rather than just supporting Labour.
But in a statement which will offer some comfort to Miliband he pledged not to cut off the unions' funding for Labour in the 2015 general election campaign.
"We will not cut our noses off to spite our face," he said.
"We want a Labour victory in 2015 and I'm sure the executive will want to give some of our money - the cleanest money in British politics - to ensuring the general election is a fair fight.
"We owe that not just to ourselves but to the health of our democracy ,which otherwise becomes a rich person's reserve."