Ed Miliband is under pressure to take radical steps to curb trade union influence over Labour amid the ongoing row over alleged ballot rigging by its biggest union donor.
Caps on spending in candidate selections and elections to senior party positions are among "significant" reforms promised in a speech due to be made on Tuesday by the Labour leader.
He said at the weekend that he wanted to "mend" the relationship to give more influence to individual members but ruled out cutting formal ties despite a bitter public row with Unite general secretary Len McCluskey.
Among changes reported to be on the table is the use of "open primaries", where all voters in a constituency can choose a general election candidate from a party shortlist, not just Labour members. But former deputy prime minister John Prescott called on him to drop opposition to more state funding for political parties to stop unions - and wealthy Tory backers - "dictating what they think is the position". And ex-cabinet minister Lord Reid said the row was based on an "ideological battle" over the future of the party.
The Conservatives also sought to keep up the pressure over the claims - fiercely denied by Unite - that the union is attempting to rig selections in favour of its preferred candidates in at least 41 seats. The row has plunged Mr Miliband into the most testing challenge of his leadership - with Tories exploiting it to press home claims that he is too "weak" to stand up to significant financial backers.
Labour deputy leader Harriet Harman insisted allegations that individuals were recruited to Labour without their knowledge under a now-axed scheme allowing unions to pay subscriptions were confined to Falkirk.
Evidence from an internal inquiry into the halted selection - which led to the suspension of two senior figures including the Unite candidate - has been passed to the police for investigation, infuriating Unite. But Ms Harman conceded that there were also "a number of constituencies where for different reasons issues are being looked into".
Conservative Party chairman Grant Shapps demanded Mr Miliband reveal which they were and publish Labour's internal report into Falkirk. He told him in a letter: "Until you reveal how deep this scandal goes, people will draw their own conclusions about your ability to lead the country." The Tories said he should re-run previous selections and "refuse to take any more money from the unions until the funding system is fixed".
Mr McCluskey kept up his assault on the leadership over Falkirk at the weekend, warning Mr Miliband to "step back from the brink of a ruinous division" and stop "playing into the hands" of the Tories. He said he would not apologise for efforts to "reclaim Labour" from an "out of touch elite" but renewed his insistence that Unite was not guilty of any criminal wrongdoing.
Mr Miliband declared that he wanted to "mend...not end" Labour's relationship with the unions, including ensuring selections were "always fair, open and transparent".