Plans to reform Labour's historic link with the trade unions will be debated by the party's national executive today amid warnings that the changes are "not a done deal".
Under the proposals, the electoral college system for leadership elections - which gives a third of the votes each to the unions, rank and file party members, and MPs and MEPs - will be scrapped for a system of one member, one vote.
Labour leader Ed Miliband made a late change to the proposals last night to avert some MPs' concerns they could reduce the choice of candidates and give the impression of a "coronation".
Would-be leaders will now require the support of 15% of MPs to get on the ballot paper rather than the 20% put forward by former Labour and union official Lord Collins in his reform blueprint.
A number of MPs felt the higher threshold could narrow the field too much, pointing out that if had been used in 2010, only Mr Miliband and his brother David would have been able to stand.
Under the new proposals, contenders would need 39 nominations and as many as six candidates could go forward for a ballot of party members.
Mr Miliband said: "This strikes the right balance between protecting the role of MPs and ensuring a diverse range of candidates going forward."
The proposals received "overwhelming" support at the Parliamentary Labour Party meeting which was attended by Mr Miliband and Lord Collins - who he tasked with overhauling the relationship in the wake of the public row over the selection of an election candidate in Falkirk.
If they are approved by the NEC, the changes will be submitted for final approval at a special conference on March 1 in London.
Under the Collins proposals, individual trade unionists will no longer be automatically affiliated through the payment of the political levy by their union, but they will be able to take part in elections if they choose to join a new category of affiliated members for a fee of just £3.
Mr Miliband said: "This is about completing unfinished business from the past 20 years and creating a One Nation Labour Party built on the principle of one member, one vote.
"And it is about letting the people back into our politics: giving people a real choice and a real voice in our party; changing Labour so that Labour can change our country."
The Collins report, seen by the Press Association, reads: "Trade unions and other affiliates should continue to have a collective constitutional role inside party structures, but on a more transparent basis.
"After a transitional period of five years, affiliation fees shall only be accepted on behalf of levy payers who have consented to the payment of such fees.
"At that point, the scale of a trade union's collective affiliation shall be governed by the number of levy payers who have consented to the payment of affiliation fees."
A section headed "closer relationship with trade unionists" says: "Levy paying trade unionists should have the ability to become affiliated supporters and to receive certain individual rights, by signing up to Labour values and providing the party with verifiable personal details.
"These individual rights should include the right to be attached to a CLP (constituency party) and to vote in leadership elections. They will not be able to represent the Labour Party or to participate in the election of party representatives - with the exception of primaries and leadership ballots - unless they join as full members.
"Party systems should be in place to enable a new category of affiliated supporters to be established before the end of 2014."
Lord Collins said in his report that the current electoral college for leadership elections should be replaced in party rules by a new system based on the principle of one member, one vote (Omov).
"Multiple voting in leadership elections should be ended. The eligible electorate should be composed of members, affiliated supporters and registered supporters.
"Members of affiliated organisations who are not already party members may take part in the ballot if they register with the party as affiliated supporters.
"This will require them to declare their support for Labour values, provide the party with personal contact details and be on the electoral roll.
"Individuals who are not already party members or members of an affiliated organisation may take part in leadership elections by registering with the party as a supporter. This will require them to declare their support for Labour values, provide the party with personal contact details, be on the electoral roll and pay the party a fee."
Responsibility for nominating and shortlisting leadership candidates would remain with MPs.
Mr Miliband said these were the biggest changes to who could become involved in the party since its formation.
Many union officials and Labour MPs have been unhappy that so much time and energy has been spent over the past few months on discussing the link between the two wings of the movement, which some believe was unnecessary.
GMB leader Paul Kenny said the reforms were "not a done deal", while other union officials have said the "devil is in the detail" of the Collins report.
Some in the party fear that the reforms will cost Labour millions of pounds in union affiliation fees, but the leader insists they will allow the party to engage more closely with trade unionists, who are currently affiliated automatically unless they choose to opt out.
Mr Miliband told MPs: "Of course change has a risk, but we have got to build a mass party, we have got to have a strong grassroots."