GMB union warns of sharp fall in Labour funding
The GMB union has told the BBC that the Labour Party risks a sharp fall in funding, as a result of the changes outlined by its leader, Ed Miliband.
Mr Miliband has said he wants to end the automatic "affiliation" fee paid by three million union members.
GMB leader, Paul Kenny, said: "I don't think they have thought through the implications of these proposals."
The union is expected to ballot its members over the issue some time in the autumn.
Labour's move came after the Unite union was accused of trying to rig the selection of the party's parliamentary candidate for Falkirk.
The GMB union said its likely that it will ballot those members who pay into its political fund to "see if they want part of that money to go to the Labour Party."
Mr Kenny warned that such a ballot could lead to a plunge in the number of members opting to pay - possibly by as much as 90%.
That would translate into a cut in funding to the Labour Party from the GMB from around £2m today to around £300,000.
"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," Mr Kenny said.
The GMB has almost 620,000 members across various sectors. It was a founding member of the Labour Party and is its third largest affiliate.