Shadow chancellor says he is not sure Corbyn will increase share of vote due to ‘numbers prevented from voting’

Polls have closed in Labour’s bitter leadership election, with the shadow chancellor, John McDonnell, predicted that Jeremy Corbyn will comfortably defeat his rival, Owen Smith.

McDonnell said, however, that he did not think it was certain that Corbyn would increase his share of the vote, as many had predicted. “I think it’s going to be really tough to get the 59.5% that we got last time around because of the numbers that have been prevented from voting,” he told a meeting of supporters in Clapham, south London, on Tuesday night.

“Some will argue if we dip below the 59.9% that somehow Jeremy’s mandate has lessened. If we win, no matter what, his mandate is still in place.”

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The result of the ballot will be announced shortly before midday on Saturday.

Ahead of his anticipated victory, Corbyn has agreed to enter mediation talks with senior figures in the party about new ways to constitute the shadow cabinet. The talks are likely to involve the leader and his office, the party’s chief whip, Rosie Winterton, the parliamentary Labour party’s chairman, John Cryer, and the deputy leader, Tom Watson.

McDonnell was sharply critical of the party’s national executive committee (NEC) for excluding people who joined the party since January from taking part in the leadership vote.

“The ruling out of 130,000 members was a disgrace, to be honest, an absolute disgrace,” he said, in comments reported by the Huffington Post. “And the way it was done was even more disgraceful.

“We lost, I think, a large group of people who were Jeremy Corbyn supporters. I think never again should we allow that to happen.”

The NEC members agreed during the meeting that there was no evidence of a “rigged purge” of Corbyn supporters, with the leader himself thanking staff for their work.

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They also offered their backing to the general secretary, Iain McNicol, who has faced reports that Corbyn’s team were preparing to try to remove him. The Guardian understands that Unite’s Jennie Formby, who some have suggested could replace McNicol, told the room that she had no interest in becoming Labour’s general secretary.

McDonnell also criticised Watson’s attempts at the NEC meeting to return to the electoral college system of voting, excluding registered supporters and giving more power to MPs and the trade unions.

“What that would mean, as it was before ... the parliamentary Labour party gets a third of the votes,” he said. “It will mean for every individual MP, their vote will mean 3,000 or 4,000 of ours. That is unacceptable.”

The NEC also agreed to toughen up Labour’s stance on internet abuse during the crucial meeting on Tuesday and members must now explicitly promise “to act within the spirit and rules of the Labour party in my conduct both on and offline, with members and non-members”.

Many of the NEC members want MPs to rally around Corbyn if he wins. Alice Perry, an Islington councillor and NEC member whose own proposals to improve representation in the Labour party were passed, said: “The NEC meeting was a positive step forward to build a Labour party that’s ready for the challenges of the future including the possibility of a snap election.

“It’s time for everyone to unite, to get behind our leader and take the fight to the Tories.”

On Tuesday night Smith wrote an open letter to Labour party members and supporters about the contest, which he admitted many had not wanted. His team insisted the letter was not intended as an admission of defeat.

“This has been a long and bruising contest, and I know many of you didn’t want it to happen,” he wrote. “But the truth is it had to happen. Our party is at a crossroads, and the choice we face is between renewing our party to pursue unity and power, or satisfying ourselves with ongoing division and opposition.

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“Unless we have a radical, credible opposition to the Tories then we won’t be able to stop them – now or at the next election. That’s the straight talking, honest truth. I regret the state we are in but I don’t regret being the one to say it.”

On Tuesday night Corbyn’s team accused Smith of making little attempt at reconciliation. “What surprised me in the last few days is that we have not had any calls for unity from Owen Smith,” a spokesman said. “Normally at this stage … you say that people should respect the outcome and work together.”

Speaking as the polls closed, Smith said his campaign had 5,000 volunteers and had spoken to 300,000 Labour supporters. “I’m incredibly grateful to all of the Labour members, supporters and trade unionists who have voted for me in this election,” he said.

“Setting out a vision for Labour’s future, to end austerity, invest in our NHS, build the 300,000 new homes a year we need to end our housing crisis, boosting workers’ rights and pay through a real living wage. I’m incredibly proud of the work we’ve already done and will continue to do for the Labour party and the people we seek to serve.”