Members of the GMB union are voting today on whether it should continue to financially support the Labour party.
Labour get £1 million a year in affiliation fees from the union, and up to £0.8 million a year in donations to constituency parties, but the group's congress will today evaluate whether the money is being well spent.
A GMB spokesman told politics.co.uk the debate was not solely about public sector pay.
"Various constituency parties get 20 grand a year from us," he said.
"And congress is asking what we get for the money. Is it worth it?"
The votes will come in a series of motions from various GMB branches, all concentrating on the link between Labour and the union.
The debate is expected to take about an hour and a half, with results expected around 17:00 BST.
Labour is facing a severe financial crisis and any move by its main funding route - the unions - to block further donations will put the party in a perilous position.