Labour leadership elections are to be thrown open to thousands of new "registered supporters" under reforms that will water down the influence of the unions.
The move will give people who are not members of either the Labour Party or its affiliated unions a say in the choice of future leaders.
The plan is being considered by the party's decision-making National Executive Committee on Tuesday and is expected to be approved by the party's annual conference in Liverpool next week.
It will involve the creation of a "registered supporters" scheme for people who are interested in the Labour Party but do not want to become a fully-fledged member.
Labour insiders hope that tens of thousands of people will sign up. Registration will be free.
The supporters will be allowed to take part in future leadership elections, with their votes being counted as part of the unions' section of the three-way electoral college.
The other two sections are MPs and MEPs, and party members.
The proposed change is regarded by Ed Miliband - who was himself elected last September thanks largely to the support of union members - as an "historic" move.
It is part of the Refounding Labour organisational review designed to make the party more outward-looking.
The package of reforms also includes scrapping elections to the shadow cabinet and will put an end to multiple votes for those with membership of more than one part of the electoral college.