Labour’s ruling national executive committee is braced for a fierce row between Momentum and trade unions over future leadership rules on Tuesday, after last-minute proposals were tabled for radical party reforms.

The party’s democracy reforms will comprehensively expand the power of members, including giving them power over local groups – a move that has angered centrists.

But leftwingers have also been angered by new proposals to strengthen the grip of the unions on selecting Jeremy Corbyn’s successor.

Labour’s general secretary, Jennie Formby, emailed NEC members on Saturday night to detail the final proposals from the democracy review carried out by Corbyn’s political secretary, Katy Clark.

Sources from both sides of the party claimed they had been blindsided by some of the new proposals, which had not been discussed in previous meetings.

Leftwing members of the NEC fear the plans to change the leadership rules would fail to hand more power to members, a cause they have long championed.

The leadership changes – a compromise understood to have been hammered out by the five major trade union leaders last week – would in effect remove the ability of the most leftwing members to reach the ballot, but would also be likely to exclude anyone from the party’s centre.

A senior party figure called the new system a “purge of the Chrises” – saying it would rule out either veteran leftwinger Chris Williamson, or centrist Corbyn critic Chris Leslie, from standing, “but no one in between”.

Momentum reacted with alarm to the leadership proposals and said a similar system in 2016 would have kept Corbyn off the ballot paper.

“The proposal to increase the leadership threshold makes it harder for a socialist candidate to get on the ballot in the future and completely misjudges the mood of the membership,” a spokesman said. “By retaining the parliamentary party’s veto over who gets on the ballot and who the membership are allowed to vote for, it gives undue influence to a tiny minority within the party.

”We saw how damaging this was in the last two leadership elections, and if there were a leadership election tomorrow using this system a candidate like Jeremy Corbyn would likely be kept off the ballot.”

NEC members from the party’s centre-left are likely to back such a compromise. “I think it would have a good chance of passing,” one source close to the NEC said. “It’s a good compromise.”

However, members of the leftwing slate on the governing body are likely to furiously oppose the proposals. “If this is proposed as part of the democracy review there will be absolute outrage amongst the membership,” one NEC member said. “For the past three years, members have fought tooth and nail for a more democratic party only to be rebuffed time and again.

”The democracy review was supposed to represent the desires of members, thousands of whom spent their evenings attending meetings and writing in submissions. If all that effort is wasted and the democracy review ends up making the party less democratic, the members will be in open revolt.”

The party’s proposals say that for a candidate to get on the ballot, they must be nominated in one of three ways. Candidates may get nominations of 10% of local parties plus at least 5% of MPs, or from at least three trade unions comprising 10% of affiliated trade union membership plus 5% of the MPs. The third option is 10% of MPs alone.

But after what one Labour source described as “frantic shuttle diplomacy”, between union general secretaries and Corbyn’s office, another option has been tabled, under which an MP would only make it onto the ballot paper if they had secured the backing of 10% of MPs, 5% of local parties, and at least three Labour affiliates – where at least two are trade unions making up 5% of the affiliated membership.

That complex system is likely to radically reduce the number of MPs who would gain enough nominations to reach the ballot paper, though it would still mean MPs were no longer the sole custodians of who can become a future Labour leader.

Corbyn critics are concerned about a series of other rule-changes. These include preventing Labour council groups from entering into coalitions, or publishing their own manifestos at general elections, without the approval of a new committee, elected by members and unions.

Another includes allowing constituency Labour parties to switch to conducting all their business through all-member meetings, instead of committees of elected officers.

When an NEC member stands aside in future, the rule-changes would allow their fellow constituency representatives on the NEC to anoint a successor – rather than the next most popular candidate in the last set of NEC elections stepping in.

When staunch Corbyn ally Christine Shawcroft resigned over the antisemitism row earlier this year, she was replaced by the centrist Eddie Izzard, to the irritation of some of his NEC colleagues, because he had won the closest number of votes at the last election.