Jeremy Corbyn is expected to bolster his dominance of Labour’s key decision-making body on Monday with the election of three leftwing candidates, paving the way for party changes that may include the reselection of MPs.

The three new positions representing party members on Labour’s national executive committee have been created in response to the surge in Labour membership.

The comedian Eddie Izzard is believed to have the most promising chance of snatching one of the seats from the party’s “left slate”. But it is expected there will be a clean sweep for the three candidates backed by the grassroots movement Momentum, tipping the once finely balanced body further in favour of Corbyn’s brand of Labour politics.

Momentum’s founder and chair, Jon Lansman, one of the three leftwing candidates, said he was confident he would win a place on the NEC.

“I’ve dreamt for a members-led Labour party. I think, I hope, that’s what we’ll have,” he told the BBC’s Pienaar’s Politics. “Members will have nine out of a 39-member executive, still under a quarter, but much better representation, a reward that 600,000 members achieved such a fantastic turnaround in the general election.”

The balance of the NEC has been in Corbyn’s favour since the unexpectedly positive result in last June’s election but the changes should give him a large majority on the body.

Previous Corbyn-sceptic rebels on the committee, including the party’s deputy leader, Tom Watson, have made it clear they are no longer prepared to challenge Corbyn’s authority and believe he has won the right to make the changes he desires.

A review of party democracy, led by the former MP Katy Clark, is set to examine whether MPs should face a mandatory re-selection triggered by party members and local affiliates, which include trade union branches.

Sources close to the NEC said there would be little the committee could do to shield MPs should the leader’s office decide to press ahead with such a measure. Two influential unions, Unison and GMB, are understood to be sceptical about backing the change.

“The leader’s office could do whatever they wanted, technically, but the unions are still extremely influential,” one source said. “It will be interesting to see how people like Lansman get on with the unions; they don’t always agree on issues and the unions will still be able to outvote them.”

Committee sources said MPs should start preparing now that re-selections were a possibility, though not a certainty. “We have to start saying to MPs now it’s possible, so you have to make sure if you do face mandatory re-selection you are able to win it,” a source close to the committee said.

“They are in the strongest position possible if they go out and engage with members regularly. They can sign up people who support them. Often the agitators are only a handful of individuals.”

The source stressed future mandatory re-selection was not anywhere near a certainty, hinting Corbyn himself was not wedded to the change. “We could be in government within the year; we do not want to look like a ragtag bunch doing factional infighting, we want to be a government in waiting,” the source said.

Stephanie Lloyd, the deputy director of centrist Labour pressure group Progress, said Lansman’s election to the NEC would be “a nightmare.”

She said: “You only have to look at his determination to bring in mandatory resection of Labour MPs; his recent calls to redo every London councillors selection with only months before polling day; how he stopped all democratic discussion on Brexit at Labour party conference; and how he runs Momentum: the end always justifies the means,. In 2017 he unilaterally changed Momentum’s rules without any democratic approval.”

One key area of dispute that may emerge from the review could be a proposal to abolish Labour’s national policy forum, to be replaced with a body that allows more members to vote on policies.

Corbyn has been a strong supporter of giving members more direct influence over the party’s policy direction. Trade unions are likely to be cautious about any move that would dilute their influence, however.

Such a change could create difficulties for Corbyn, too, as members are likely to diverge from the leader’s office on some key issues: for example, the overwhelming number of members are more solidly pro-EU than the party’s current official Brexit position, which is to leave the single market.