When Angela Rayner, Labour’s deputy leader, stood up to huge cheers to address her party’s conference on Saturday, the slogan “Stronger Future Together” was projected on the big screen behind her and all around the vast hall.

Before what had been billed as Keir Starmer’s defining conference, when he would reveal the real Keir to the nation, the party leader had also spoken last week of the need for togetherness in the party in a 35-page pamphlet spelling out his vision for Britain.

“In recent decades, the legacy of the 1997 Labour government has become contested to the extent that the party has at times felt like separate families living under one roof. This has been harmful and alienating,” Starmer wrote.

But before most delegates had even arrived in Brighton, hopes of Starmer’s party hanging together for four days by the seaside and focusing on attacking the Tories, as they struggle to deal with the threat of fuel shortages and empty shelves in supermarkets, had been horribly shattered.

Before Rayner’s speech her friends were briefing that she was furious with Starmer. The deputy leader, they said, was enraged that her boss had allowed the start of the annual gathering, potentially the second last before a general election, to be hijacked by an unnecessary row over changes to rules on how future leaders should be elected, to which she was opposed.

Rayner had to spend much of Saturday morning, when she should have been preparing for her big conference moment, holed up in an emergency meeting of the national executive committee in Brighton’s Hilton hotel, trying to amend Starmer’s plans into an acceptable form, and forge compromises on other rule changes with the unions.

“She was against Keir’s main plan,” said a source. “But she also had to try to broker some deal before the whole party fell apart. She was very cross because she had spent months talking to the unions about the serious economic plans we wanted to launch at conference, only for them to be buried in the media under coverage of this mess.”

On Saturday, the centrepiece of the Starmer plan – to weaken the role of the 400,000 party members and strengthen that of MPs by returning to the old electoral college system for electing leaders – was pulled hours before the gathering opened after the unions ganged up to oppose it. The left saw it as an anti-democratic stitch-up and an attack on individual members who had elected Jeremy Corbyn that was designed to prevent another from his wing of the party ever becoming leader. The conference mood had been set before a single motion had been debated.

Not only were the leader and his deputy at odds, the unions were defiant in victory and the media focused on little else. But across the party, from top to bottom, there was disbelief at how the fiasco could have been allowed to happen.

Unlike previous leaders, from Neil Kinnock over his battle with the Militant tendency to John Smith’s fight to deliver one member one vote (Omov), and Tony Blair’s confrontation with sections of his party over the rewriting of clause IV, Starmer had just taken on the left – and lost.

A senior shadow cabinet member said: “It is has been a very, very, very bad start. The problem is that they just don’t listen to advice. Why did they try to introduce these rule changes at all and why did they then do so before checking first that they could land them? It really is unbelievable.”

Another senior figure said that Starmer had mentioned the plan only briefly in a meeting of the shadow cabinet last Tuesday, and in such a way that many of those present probably didn’t clock what he meant.

“It is pretty clear that what they were trying to do is to get a win over the left for Keir, a Kinnock moment, and by doing it so late and quietly, they hoped opposition could not build in time.”

Another senior source said there was widespread suspicion that Peter Mandelson had had a role in suggesting the idea, something he denies. “There is a view that Peter was involved – if not directly, then at once or twice removed,” said the source.

Rows between Labour leaders and their deputies have happened before. Two years ago, the opening of the conference was wrecked by a clumsy attempt by Corbyn supporters to abolish the post of deputy leader, then occupied by Tom Watson. Such was the uproar at the way the plot had been hatched in secret that the idea had to be swiftly abandoned – but not before much bad blood had been spilled.

On Saturday senior figures said Starmer’s disastrous start was worse than in 2019. One said: “It is difficult to upstage what happened two years ago but Keir has had a bloody good go, hasn’t he?” Another senior source, when asked if the leader could pull the conference back from the brink, was pessimistic: “I really don’t think so. His authority is shot.”

Outside the conference hall there were cries of “Stop the purge” and “Starmer out. Socialism in”. Members of the frontbench desperately tried to sell the rule changes that were in the end agreed, including one that makes it more difficult for local parties to deselect their MPs, as a great success. “Despite all the noise,” one shadow minister said, “Keir has made his mark. He has locked out the hard-left in MP selections.”

But privately, even loyalists were in despair. The Brighton gathering had been billed for months by Starmer’s team as the moment he would truly define himself in the minds of the electorate, after criticism of his lack of eyecatching policies and Labour’s failure to overtake the Tories in the polls, despite successive government crises.

Despite all this, moods can change fast. There is still an opportunity for the party to unite behind a set of ideas and policies that will allow delegates to leave Brighton on Wednesday in upbeat mood.

Conference organisers say Starmer’s speech will be packed with announcements. Ed Miliband, the shadow business and energy secretary, is going to push his foot hard down on the “green new deal” with bold plans to modernise and green the steel industry in the interests of saving the climate and preserving jobs.

Miliband has been making the case in shadow cabinet for a president Biden-style stimulus to decarbonise industry, create jobs in low carbon industries, and rapidly retrofit homes. There will also be more announcements on security in work and training and education for young people.

However, Starmer will wrap up the conference on Wednesday with even more now riding on his speech than before.

But speaking out against Starmer’s leadership is no longer regarded as shocking and on the left it has become commonplace. The attention-grabbing policies have yet to be revealed.

On Saturday in a blistering article in the Observer the former shadow chancellor John McDonnell says he no longer feels able to play the loyal elder statesman and instead has no option but to speak out. And on Saturday night, Starmer’s predecessor Jeremy Corbyn hinted that the left could organise against Starmer because of their feeling that he has betrayed them.

The words “Stronger Future Together” may be plastered across every surface in Brighton, but last night that slogan appeared less apposite than at any time since Starmer became leader.