Ed Miliband has accepted his move against the trade unions is a "gamble", following the latest veiled warning from Unite's Len McCluskey.
The Labour leader used an article for the Sun on Sunday newspaper to accept his proposed switch in the way Labour is funded via the trade unions will "cost us money".
Last week Miliband proposed changing the trade unions' affiliation with the Labour party to a system where individual members opt-in to funding the party.
Union chiefs have offered mixed responses. Len McCluskey, the leader of the Unite union which has donated over £8 million to Labour since Ed Miliband became leader, has voiced a careful warning to Labour, calling the move a "gamble".
"If we are to go out and convince thousands of working class men and women that they want to sign up to be associate Labour party members they will not be interested in the rulebook, or even the history," he told the annual Durham miners' gala.
"They will want to know – will Labour make a difference?
"Will the next Labour government reverse the present coalition's disastrous policies? Will it be different not just from Cameron and his crew but from the Blair-Brown years as well?
"If we can say 'Yes, Labour has learned, and Labour is on your side' then this scheme will work.
"But if our people – our members – are unclear as to the answer then no amount of persuading will get them to sign up."
Miliband's response has been to acknowledge that the changes "will make financing our party more difficult".
"They will cost us money and we will be competing with the Conservatives, whose wealthy donors have much deeper pockets," he wrote.
"But if it’s a gamble, that's because I’m taking a bet on YOU.
"I believe we can begin to turn this country round if we work together and better involve working people in what we do."
Labour's former London mayor Ken Livingstone suggested the public were deeply cynical regardless, however.
"We got into an arms race with the Tories on how much we're going to spend in a general election," he told Sky News' Murnaghan programme.
"And most people don't believe a word we're saying."
Former Labour minister Chris Mullin also voiced caution at a Devon literary festival.
He was quoted by the Sunday Telegraph newspaper as predicting a "huge collapse in financial support for the Labour party" which would result in "a one-party state for years to come".
Meanwhile the Conservatives are reportedly considering a bid to include funding options for the Tories on trade union membership forms.
"We don't accept his attempt to divert your attention by saying this is now an issue about party funding," party chairman Grant Shapps told Sky News.
"It's about union influence on the Labour party selecting candidates."
Miliband's funding reforms were triggered by a scandal over the Unite union "effectively fiddling" the selection process in 40 constituencies, Shapps claimed.
The Labour party has referred the allegations in the Falkirk constituency to the police.