Ed Miliband wants Labour leadership election rule change
The Labour Party is considering changing the way its leader is elected as part of talks about the future of its relationship with the unions.
The BBC's Ross Hawkins says Ed Miliband was proposing change - with "one member, one vote" among the options.
But trade union sources have accused Labour MPs of resisting reforms over fears they will lose influence.
The reforms could spell the end of the "electoral college" system for choosing Labour leaders, including Mr Miliband.
The system sees party members, MPs and MEPs, and organisations affiliated to the party like unions get a third of the vote each.
A union source told the BBC that Labour MPs were resisting the prospect, anxious that their votes would be diluted in future as they would no longer represent a third of the college.
A senior Labour source said: "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."
Earlier, the GMB leader Paul Kenny said talks led by a former party official - Ray Collins - had broken down. This has been rejected by Labour.
The party source said: "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."
There will be a special party conference in March to discuss the changes.
Conservative MP Priti Patel said: "As we have known all along, these so-called 'reforms' are a sham. Nothing has changed."
Mr Miliband wants to reform his party's historic link with the unions, with individual union members given the choice of opting in to join the party, rather than being automatically affiliated.
It is understood the unions were not willing to change voting arrangements for Labour conferences, or for the party's national executive.
Instead, they have proposed a formula which would see new union members, and eventually existing members, specify that they wanted to be affiliated to Labour.
It was thought that this would be "radical" enough to satisfy Mr Miliband.
The BBC's Iain Watson said the unions were on the verge of agreeing that the electoral college should be abolished - not for conference votes, just for the leadership election - so all members, including the 'opted-in', union-affiliated members, would get one vote each.
Labour sources say the leadership election is not the only stumbling block - there are also technical issues, such as concern that accessing union membership lists could breach data protection laws.
The GMB has already decided to drastically cut its funding of Labour in protest at the reforms, and other unions could follow suit if there is no agreement.
Lord Collins is due to report to a meeting of Labour's executive on 4 February, ahead of the special conference in London.