Members of the public registering as individual Labour supporters could be given a vote in leadership elections and possibly at party conference.
The proposals for reforming the party's structure and culture, to be outlined by leader Ed Miliband and the chair of the national policy forum, Peter Hain, are potentially far broader than expected, according to Hain.
"They are designed to give Labour the chance to leapfrog the other parties and become a new party for a new political age," he said. "Politics has changed, but parties have not.
"It is about being a more pluralist party offering a more pluralist politics in tune with a more pluralist Britain. We need to be the party most willing to extend our reach to broad social movements beyond the immediate boundaries of Labour. This will make Labour's borders more porous."
Hain, who said the plans could represent the biggest change to party organisation since its formation, added: "The halving of union affiliated membership means our reach into the workplace has diminished, and we have to address that. There are hundreds of people out there who could be attracted to signing up as supporters, but do not see themselves as joiners or full members. These ideas are designed to create a very big debate, with the idea of changes going to conference this September."
How registered supporters could be involved in leadership elections will not be detailed tomorrow, but Hain said they could be given their own section in the electoral college of MPs, individual members and affiliates. Or they could be put in the section for unions and socialist societies, indirectly diluting union influence and putting them under pressure to recruit.
A new category of affiliated consultee might give pressure groups such as MumsNet, or pensioners' groups, a formal voice in policymaking.
The idea of registered supporters has been canvassed before, but Miliband is the first leader to suggest giving them a vote.
Hain said: "We need to be a party that reaches way beyond its formal borders, including giving registered supporters, a new category of membership, a vote in leadership elections. So you would not only have 200,000 party members, 2.5m affiliated members but also a new category registered supporters". Such supporters would pay a nominal £1 fee as opposed to the full membership rate of £41 annually.
Hain said the plans should be seen in the context of Miliband speaking to Saturday's TUC march against the cuts, and his decision to speak at a cross-party event with Green leader Caroline Lucas and Lib Dem president Tim Farron on the alternative vote.
Miliband will say: "The Tory-led government and its current alliance of power with the Liberal Democrats does not change my belief that there is a progressive majority in this country."
In a 10,000-word paper, Refounding Labour: a Party for a New Generation, Hain warns that "declining individual and affiliated membership has narrowed the range of voices heard within the party's discussions and reduced the chances of a voter hearing the party's policies advocated in everyday life".
Setting out the case for widening the range of voices in the party, he says unions now represent only a quarter of people in work and just 15% in the private sector. "If unions could rebuild their membership they would speak with a stronger voice in society," Hain writes. "Despite improved union recognition rights under Labour they have been unable to do so."
Union-affiliated membership has fallen by more than a half from a peak of 6.5m in 1979 to 2.7m, of whom nearly 10% voted in the Labour leadership election. Union mergers, he adds, "have significant implications for the party leadership and in the leadership election for what used traditionally to be a much more diverse sector industrially and politically. Where once there were numerous activists, in almost all constituency Labour parties now, they are now few and far between."
The paper says: "Party membership has more than halved since 1997 to just over 150,000. We are still spread thinly on the ground with a weak base from which to develop contacts in the community and build popular support. In too many constituencies where Labour's vote is small, our party barely functions."
The Hain paper says the "party has to be frank about weaknesses inherent in Labour's organisation, culture and outlook which amount to more than simple wear and tear that can be patched up with a bit of make do and mend."
He adds: "The worse we do in elections, the weaker our party organisation becomes and more daunting the next electoral challenge appears. By 2005 only 1.3% of UK voters were members of a political party down from 4% in 1983. The widespread disengagement from party politics can partly be explained by the rise in the consumer sociarty and competing pressure on peoples' time from work and study obligations to family and leisure or sport interests."