Sir Keir Starmer must use the Labour conference to show the nation he "has what it takes" to be prime minister, the leader of the GMB Union has said.
Gary Smith, who took over as general secretary earlier this year, told Sky News the Labour leader was "decent" and "committed".
But he warned the party had become "disconnected from the concerns of ordinary working class people" and must still show it can "demonstrate common purpose".
In a warning ahead of the conference in Brighton this weekend, Mr Smith urged the party to listen to the stories of working people, adding there were still some in Labour who "look down their nose".
"I think what he's got to do this week is sell his vision to the country, and demonstrate that Labour can come together with a common purpose, because if we don't have common purpose, they cannot win," Mr Smith told Sky News.
"It is up to him to demonstrate to the nation that he has what it takes to be a prime minister and I guess and that this is going to be a massive week for him", he added.
Sir Keir is currently in negotiations with trade unions over his plans to scrap Labour's one member, one vote system for electing the party leader, in favour of a return to an electoral college made up of unions and affiliate organisations, MPs and party members.
Some of the more left wing trade unions, such as the TSSA and CWU, have already said they will not back the proposals, and former shadow chancellor John McDonnell has said the changes would "rip our party apart".
But Mr Smith signalled he recognised the need for changes.
"The current situation, I am very clear, does not work for ordinary trade union members, and is part of the dis-location between the Labour Party and ordinary trade union members," he said.
The GMB boss said he hoped trade unions would be able to reach a joint position on rule changes by Saturday.
However he warned "very few of our members are giving much time worrying about what's happening in the internal machinations of the Labour Party".
"I think what we have to grasp is that there are millions of trade unionists in this country who are totally disconnected from the democracy of Labour, people who pay a political levy, and we are going to have to sort this out," he said.
Earlier, shadow communities secretary Steve Reed told Sky News "I'm not Mystic Meg and I can't tell you exactly what is going to happen" when asked if he thought the rule changes would pass.
Mr Smith, who is one of the most sympathetic union leaders to the Labour leadership, says he has not yet read the 11,500 word essay Sir Keir has written setting out his vision for the future of the party.
However, he rejected one of the flagship ideas set out in the pamphlet, which calls for the UK to become mostly carbon neutral by 2030.
"I think a lot of the debate around energy and the environment has been fundamentally dishonest," he said.
"The energy market is broken, energy is very complex, and I don't think politicians have entirely grasped that. We have a national security issue unfolding around energy, around security supplies.
"And of course, industry is going to struggle with rising energy costs. Low paid people the length and breadth of the country will really struggle as bills go through the roof, early in the new year."
"So I think it's good to have an aspiration, but you have to have a plan, and you have to have a plan for jobs," he added.
Mr Smith has said he will use the party conference in Brighton, the first he will attend as general secretary, to speak about the lives of working people.
In contrast, the newly-elected leader of the Unite union, Sharon Graham, has said she will not be attending because she wants to focus on resolving disputes on behalf of her members.