Union money is the "cleanest cash in politics," the new head of the TUC will argue today in a direct challenge to David Cameron.
Addressing the Prime Minister, Frances O'Grady will today call for more transparency from the Conservative Party, arguing that politics has been controlled for too long by those who have too much money and power.
As the controversial row over Labour's funding continues to brew, the first female general secretary of the TUC will demand that Cameron "take a long hard look in the mirror," over issues of funding.
In her first speech to the TUC Congress as general secretary, O'Grady will call for ordinary people to share in the economic recovery in a challenge to politicians from all parties to say where they stand on issues such as pay, jobs, rights at work and privatisation of public services.
As Ed Miliband remains embroiled in difficult relations with the trade union movement, O'Grady will tell the Bournemouth conference: "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," she will say.
"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.
The comments are due to come as Labour has been warned it looks like a "disunited" party because of the row over its links with unions.
Dave Prentis, general secretary of Unison, warned that people would not vote for a party which was "squabbling".
O'Grady has already urged Labour and the unions to "shake hands and move on" from their row over affiliation.