Labour leader Ed Miliband will face tough questioning tomorrow when he addresses the TUC Congress - the first time he will come face to face with activists since unveiling controversial reforms to union funding of the party.
He will take questions from delegates after making a speech at the conference in Bournemouth, in which he will put forward plans to tackle abuses of zero hours contracts.
He is expected to be asked about his plans for union members to opt in to Labour affiliation rather than being automatically affiliated, a move which has already led to the GMB cutting its funding of the party by over £1 million, as well as the party's spending plans if they win the next election.
Unions have been angered by statements that Labour would stick to the coalition's spending plans, calling on the party to promote different policies to the austerity measures which they complain have caused huge problems for millions of workers.
Len McCluskey, general secretary of Unite, urged Labour's leadership not to be "embarrassed" by its historic link with unions, amid warnings that it will be weakened by the reforms.
He told a fringe meeting at the TUC: "They should embrace it, because we are the real voice of decent working people. Our Labour leadership has to start demonstrating they are on the side of ordinary working people, with policies that takes us away from the path to poverty."
Mr Miliband will hold private talks with senior union leaders tomorrow, following weeks of in-fighting between the two wings of the labour movement.
Union officials have called for an end to the row which flared after Unite was accused of trying to influence the selection of a Labour candidate in Falkirk.
The union was cleared after an internal review, but the repurcussions have continued after Mr Miliband said he wanted to review the relationship between Labour and unions.
A special conference will be held next March to approve the reforms, but action has already been taken which will hit Labour's finances.
The GMB said it will reduce the number of its members it affiliates to Labour, leading to a reduction of around £1.1 million from January, while Unison said its money will also reduce from next year.
Mr Miliband is expected to attack the Government on the state of the economy during his speech, which is one of the most important of his leadership given the strained relationship with unions.
Mr Miliband is expected to say: "David Cameron boasts that he's fixed Britain's economy, but hard-working families know things are getting harder, not easier, for them.
"But there is one group who does feel there is an economic recovery. The privileged few in our society. They are certainly doing well out of this Government. The City bonuses are back - up 82% in April of this year alone.
"After over three years of this Government, with our economy still smaller than it was before the financial crisis, the rewards in our economy are going only to the few at the top. And that's not just unfair. It's bad for our country.
"Because an unequal recovery won't be a stable recovery. It won't be built to last.
"The only way we can have a durable recovery is with an economy that works for all working people.
"I recognise, as do you, that both workers and businesses need flexibility.
"It is how you have rightly worked to keep people working even during the most difficult moments of the recession, working fewer hours, putting jobs above pay rises.
"We need flexibility. But we must stop flexibility being used as the excuse for exploitation.
"Exploitation which leaves workers carrying all of the burdens of unpredictable hours, irregular pay, no security for the future.
"Nowhere is that more true than when it comes to zero hours contracts.
"Of course, there are some kinds of these contracts which are useful. For doctors, or supply teachers at schools, or sometimes, young people working in bars. But you and I know that zero hours contracts have been terribly misused. This kind of exploitation has to stop. We will support those businesses and workers that want to get on in life, but we will ban practices which lead to people being ground down."
The head of the TUC launched a scathing attack on the Government for trying to "divide" the country, and likened ministers to characters from The Wizard of Oz.
General secretary Francis O'Grady said Conservatives were fond of referring to PR adviser Australian Lynton Crosby as their own Wizard of Oz.
"What does that make Cameron, Osborne and Clegg? When it comes to any vision for a new economy, they are the Scarecrow, the Tin Man and the Cowardly Lion," she told the Congress.
She accused the coalition of trying to divide Britain "Thatcher-style" between those in and out of work and between top rate taxpayers and everyone else.
Unions had not been vilified in other countries as they were in the UK, she said, adding that more thoughtful Conservatives were nervous about the "war" on workers.
"They worry that attacks on unions, on ordinary decent working men and women look high-handed, cold-hearted and out of touch."
Ms O'Grady said families were facing problems which "the Eton-educated elite, with their serial holidays, hired help and inherited millions, simply haven't got a clue about."
In a reference to the row over funding of the Labour Party, she said that union money is the "cleanest cash in politics."
She went on: "Whether unions set up a political fund is a matter for members, not ministers.
"Because for too long, politics has been controlled by those who already have far too much money and far too much power.
"Half the Conservative Party's funding comes from the City. One third of its new intake of MPs are drawn from the banking industry alone and we know what happens when the super-rich get to run the tax system.
"In contrast, unions are Britain's biggest democratic membership movement of ordinary people. We are already required by law to report our membership records every year. We have more than 10 times the membership of all of Britain's political parties put together. It may even be more. The truth is, we simply don't know.
"Because political parties don't have to account for their members in the way that we have to account for ours. In fact, the Conservative Party refuses point blank to say how many members it has.
"So before he starts lecturing unions about transparency, the Prime Minister should take a long hard look in the mirror.
"We already publish our numbers. I challenge David Cameron to publish his."