Ed Miliband has tabled proposals billed as the biggest change to the party's structures for 20 years, including a registered supporters' group to vote in elections for the Labour leader and deputy leader.

The supporters' group, to be administered by local parties, would vote in the union section, so diluting the unions' influence. Unions currently hold a third of the vote in the leadership electoral college, with MPs claiming another third and constituency party members the remaining third. Tens of thousands of registered supporters would need to be recruited to counterbalance the 3.5 million people who pay union levies. However, as few levy payers actually vote, registered supporters might represent half the union section within five years.

Miliband has been forced to defer plans to change the voting powers at the Labour conference, leaving unions to keep 50% of the vote. In practice, Unite, Unison and the GMB control 40% of the vote at conference, making it almost impossible for constituencies to defeat them. Miliband has proposed a review, due to be completed in the spring, to make conference voting more democratic. Options considered include giving Labour's national policy forum a larger vote at conference.

The Liberal Democrats described the changes as fiddling round the edges. The Conservative chairwoman, Lady Warsi, said: "It's no surprise Ed Miliband has failed to curb the unions' power over Labour leadership elections and Labour conference. After all, he's only Labour leader because of trade union votes, and his party is only solvent because of trade union money. This failure to reform Labour's relationship with the unions is a real blow to Ed Miliband's credibility."

Miliband is a genuine believer in the union link, but he would like to breathe life into the relationship between the party and political levy payers.

He is also proposing to curtail multiple voting. MPs in the leadership election will only be allowed to vote in their section, and not in the party membership section of the college. Union affiliates will be able to vote twice if they are party members: once in the union section and once in the party membership section. Union leaders are likely to be pleased that they have not felt forced to give ground yet. They fear any dilution of their power at conference, believing it might open the way for a long-term programme that will see their power dismantled.

The registered supporters' scheme will be run by local parties, but it remains to be seen whether there is a group of people interested in supporting but not joining the party. Previous efforts to set up registered supporters' schemes foundered both in the Conservative party and Labour.

But if the scheme took off, some Labour officials believe registered supporters could open the way to US-style primaries.

Party sources said the unions had agreed that local parties would be able to communicate directly with union levy payers in their constituencies.

They might be expected to pay a nominal sum, rather than the full cost of membership. At present, local parties do not have access to union political levy membership lists, leading to complaints that unions are controlling their members and not letting them be contacted by leadership candidates directly. During the last leadership election, David Miliband's team complained that once a union executive decided to back his brother Ed, the union remained closed to their campaign team.

Unions have been resisting handing over membership lists to local parties on the basis that it might breach the data protection act, arguing that names and addresses cannot be handed to a separate external organisation without the permission of the membership.

The proposals are due to be discussed by the national executive, and will go before the party conference next week.

Research from academics at the University of Bristol claims the 2010 Labour leadership election did not meet the definition of a "free and fair democratic election". It claimed that the trade unions created a "block vote" in favour of their preferred candidate, Ed Miliband.