Labour presses on with 'one member, one vote' leadership reforms
Plans to reform Labour leadership elections are expected to go before the party's national executive committee this weekend.
The party is thought to be planning to adopt the one member, one vote system.
Trade unions, which account for one third of the votes under current rules, have protested about the plans.
Leader Ed Miliband first proposed a "historic" change in his party's relationship with the unions after the Falkirk candidate selection scandal.
Under the proposals, each of the roughly 200,000 Labour members will get a single vote in choosing the leader, but so will a new class of so-called associate members, who will pay a membership fee of £3 a year.
MPs and MEPs, who collectively also have a third of the votes in leadership elections at present, will be relegated to a role in short-listing candidates.'No private dinners'
Under current rules, the other two thirds of the power to elect a leader is divided equally between party members and of members of "affiliated organisations", such as unions.
Some senior Labour party figures are worried a move aimed at diminishing the unions' influence could end up handing them even more power.
Ed Miliband... is on the brink of making what could be the biggest decision of his leadership so far”
There are also concerns union leaders will use their powers to register large numbers of associate members and influence their decisions, despite the loss of their votes.
Paul Kenny, general secretary of the GMB union, said plans had not been finalised but he unions would welcome a watering down of MPs' influence in the electoral college.
"MPs are making a fuss about this on the basis that they're going to lose this golden share of the vote," he said.
"It's more like a storm in the Westminster claret glass than it is in anything in the real world."
Mr Kenny told BBC Radio 4's Today programme that unions would not accept any "watering down" of their collective voice - or of their "open and transparent" role in the Labour Party.
"We don't hide behind back doors. We don't have private dinners and hand over big cheques. Everything we do is in public."
He added: "The collective role of trade unions in the Labour Party is not up for grabs.
"If that breaks then the link with the Labour Party breaks."
Mr Kenny said the GMB had cut its affiliation fees payments to Labour to £150,000 to reflect the number of members it expected to opt in to supporting the party.
The proposed changes to Labour's union links are expected to be introduced incrementally over a five-year period.
The announcement came after Unite, one of the party's biggest donors, was accused of signing up its members to Labour in Falkirk - some without their knowledge - in an effort to get its preferred parliamentary candidate selected.
The union was later cleared in an internal investigation.