Labour leader Ed Miliband's plans to reform the party's historic link with the trade unions could deprive the party of its "meaning", a senior backbench MP has warned.
Plans to change the way the party elects its leaders, potentially including a move to a "one member, one vote" system are part of the overhaul of the relationship.
The proposals are due to be circulated to the party's National Executive Committee (NEC) this weekend, according to reports, following months of negotiations led by former Labour and union official Lord Collins.
But former leadership contender Diane Abbott warned weakening the link with the unions could leave the party as "just a bunch of Oxbridge PPE graduates".
According to BBC2's Newsnight and Channel 4 News, the plans which will be put before the NEC will end the electoral college system for choosing leaders, which gave one third of the vote to MPs and another third to the unions, with the remaining section made up by members.
Mr Miliband wants individual union members to be given the choice of opting in to join Labour, rather than being automatically affiliated.
His proposed reforms followed controversy last year over the selection of a Labour candidate in Falkirk, where Unite was accused of signing up members in the constituency so it could influence the choice.
Unite always denied doing anything wrong.
Former home secretary Alan Johnson told Channel 4 Mr Miliband planned to end the presence of "ghosts in the machine, the people who don't make a conscious decision to sign up to Labour but become our so-called levy payers and take a role in the party".
He added: "Now Ed has said that has to end, that means, the natural consequence of that is one member, one vote."
But former shadow minister Ms Abbott said: "I think we should be proud of the trade union link. I think without the trade union link the Labour Party is just a bunch of Oxbridge PPE graduates looking for constituencies to represent.
"It's the trade union link which gives the Labour Party its meaning."
She added: " It's not about having a disproportionate amount of power, but it's allowing them to have a collective voice. I think there should be reform, but I feel very strongly about defending the link."
The financial support offered by the unions was another consideration, she added.
"If we give way to the people pushing to weaken the trade union link and weaken the collective voice, what is to say that trade unions will always want to give their political finance to the Labour Party? In Scotland they could look at the SNP.
"Contrary to people saying 'Oh, Ed is in the pocket of trade union leaders', Ed Balls is pursuing a lot of policies in the future which trade union leaders won't like and we may find that they don't choose to give their money."
A decision on the proposals is due to be made at a special conference on March 1 and a senior Labour source said detailed discussions were still taking place.
The source said: "Ed has made clear he wants radical reform.
"Ray Collins has yet to publish his report and you get this sort of speculation emerging in public when detailed discussions are taking place in private.
"Ed has always been clear that the scale of his reforms mean there are likely to be consequences for other rules and structures in the Labour Party.
"He is proposing that we change the way we elect our leaders and discussions on party reform are continuing."