The British Labour Party will be plunged into “chaos” if Andy Burnham fails to win next month’s byelection in Makerfield and go on to succeed Keir Starmer as prime minister, the former shadow chancellor of the exchequer, John McDonnell, has said at a political event in Dublin.Addressing the Robert Tressell festival at the RDS on Saturday, McDonnell said if Burnham does win he expects members of the cabinet to make it clear to Starmer he must stand down, with Burnham then being handed the leadership unchallenged.If not, he told The Irish Times, there was likely to be “chaos”, with no immediate leadership challenge and Starmer struggling on until at least Christmas. He said this would shorten the time the party has to reverse its fortunes and public standing in advance of the next general election.“I can’t see anybody else getting the 80 nominations they need to run,” McDonnell said. “Wes Streeting thinks he can get that, but if you look at the opinion polling of party members his support is between five and 10 per cent, so I don’t think that’s the basis for a feasible leadership challenge. I think he has recognised that.“Keir Starmer has said he will fight any leadership challenge if there is one, but I think he’s got more sense than that and I think there will be a lot of pressure on him to put the party first rather than his own career.”McDonnell was first elected to parliament in 1997 and twice sought to run for the leadership. “It’s just the nature of politics. Once you lose the trust of the electorate, it’s almost impossible to regain it. And I think that’s what’s happened. On the doorsteps, there’s such anger towards him,” he said. “When he was elected, my view was: ‘Give this guy a chance, let’s see what he can do.’ Because when ... Jeremy [Corbyn] was leader we seemed to face a coup every month. But in every opinion poll, his rating is absolutely dire, and on the doorsteps, we’ve never come across this level of opposition to a leader before.”[ Tinderbox Britain: violence over Henry Nowak case stirs fears of another summer of disorderOpens in new window ]He said Burnham could have had the support of the party’s left in the 2015 leadership race but had made himself unsupportable by abstaining in a vote on a welfare bill, after which Corbyn won decisively.“I think Andy Burnham has moved on from what he was then, exactly because of his experience in Manchester where he’s done an extremely good job,” McDonnell said. “He’s demonstrated how he can bring the community together. I think that’s why he’s become so prominent, and why he’s been so popular.” McDonnell said he not yet decided whether he would support Burnham, but suggested he looks the most likely to give Labour a chance of fighting off the electoral challenge it faces from both Reform on the right and Greens on the left.First, however, the Manchester mayor needs to win the byelection which, McDonnell said, “is touch and go” based on the opinion polls. “We’re running neck and neck with Reform – and the danger of losing Makerfield to Reform is that it would give [them] huge momentum when we’re at such a dangerous moment in the UK where we could end up with a Reform government.”