The size of the challenge facing Ed Miliband was thrown into stark relief this morning, after a prominent union leader warned just ten per cent of his members would be likely to join Labour.
Miliband took the biggest gamble of his time as leader yesterday when he confirmed that only individual union members who actively opt-in to supporting Labour would contribute to the party, rather than the current system of an automatic 'affiliation' fee paid by three million union members.
"That would require them to become associate members of the Labour party and I'm not sure that's what those that pay the political levy want," GMB general secretary Paul Kenny told the Today programme this morning.
"We will now have to ballot out members. I think we'd be lucky if ten per cent of our current membership levels say yes, they want to be members of the Labour party."
GMB is the UK's third largest trade union with 600,000 members.
If Kenny's predictions prove accurate, the union's funding to labour could drop from £2 million to less than £1 million as early as September, when the union plans to ballot its members.
"This is an absolute watershed moment - the changes they are about to do will fundamentally change the party's relationship with trade unions and their members," Kenny said.
The Labour's leader's speech was generally well-received yesterday, but Labour MPs are concerned at the ramifications of the change on the party's financial arrangements, especially given it is currently in debt.
Writing in the Mirror this morning, Miliband used his decisive action on union funding to get on the front foot against David Cameron, in an attack he is likely to use at today's PMQs.
"When we had a problem in one of our constituencies, we acted swiftly and thoroughly," he wrote.
"A year ago David Cameron faced the dinners for donors scandal where wealthy Tory backers were given access to Downing Street in return for huge sums.
"He still has done nothing about this. His party still relies on getting half its money from the bankers and the City.
"We cannot go on like this. I have acted to build a better Labour party and a better politics for Britain. The time has come for David Cameron to act too."
The changes to union funding come after allegations that Unite signed up members to Labour without their knowledge in order to secure its candidate in Falkirk.