Keir Starmer will on Wednesday repudiate the legacy of Jeremy Corbyn and accuse the government of being “lost in the woods” in a keenly awaited conference speech that will put addressing Britain’s mental health crisis at the heart of his pitch to voters.

In a sign of the significance the Labour leader puts on the issue, Starmer will promise a Labour government will make mental health treatment available to everyone who needs it within a month, and create drop-in mental health hubs aimed at children and young people.

The pledges will be set out in his first “in-person” address to his party’s conference in Brighton which he hopes will show Labour is “back in business,” and has a programme for tackling “the chronic problems revealed by Covid”.

Labour believes the mounting fuel crisis is stoking voters’ concerns about Boris Johnson’s competence, with the government assailed on a number of fronts and even the Tory-leaning press accusing Downing Street of having lost its grip. “The shine has come off,” said a party spokesperson.

After a conference marred by divisions over party rule changes and the resignation of left-winger Andy McDonald from the shadow cabinet, Starmer’s speech will seek to present Labour as a government in waiting.

Asked on Tuesday whether he would prioritise party unity or winning the general election, he told the BBC it would be “winning”.

Though Starmer is not expected to mention his predecessor by name, the tenor of the speech will be aimed at underlining the stark differences between the two men and their policies.

A Labour source said: “The speech will be a demonstration of the way the Labour party has changed. It will be a clear indication that Labour will never again go into an election with a manifesto that isn’t a serious plan for government.”

As Boris Johnson on Tuesday appealed to the public not to fill their cars with petrol unless necessary, Starmer will seek to contrast the government’s chaotic approach with the scale of the challenges facing the country.

He is expected to say: “I see the government lost in the woods with two paths beckoning. One path leads back where we came from. None of the lessons of Covid are heeded. The divisions and flaws that were brutally exposed by the pandemic all worsen.

“But there is another path down which we address the chronic problems revealed by Covid with the kindness and the togetherness that got us through.”

He will call mental health “one of the urgent needs of our time,” and promise that a Labour government will tackle it on a number of fronts.

As well as setting a new NHS target guaranteeing treatment within a month, and creating “hubs” for young people, Starmer will pledge to recruit 8,500 new mental health staff and ensure every secondary school has a full-time mental health specialist.

“We’ll make sure children and young people get early help, putting specialist support in every school and a mental health hub in every community,” he will say. “This is prevention in action. Helping young people, looking after their wellbeing.”

Labour calculates the package of measures would cost GBP1bn a year by the end of the next parliament, allowing one million more people to be treated annually than the government is currently planning to help.

It would fund the policies with two tax increases already announced – the charging of VAT on private school fees, and the closing of loopholes used by private equity investors.

The Labour leader succeeded in pushing through a series of radical changes to Labour’s internal rulebook on Sunday in the face of furious objections from the party’s leftwingers.

When McDonald resigned as shadow employment rights minister on Tuesday, he claimed Starmer had repudiated the “10 principles” on which he won the leadership, and exacerbated the party’s divisions.

McDonald abruptly stepped down on Tuesday, saying Starmer’s office was asking him to reject union calls for a GBP15 an hour minimum wage, which he felt he could not do.

Conference delegates backed a motion calling for the GBP15 minimum wage in Brighton on Tuesday, but Starmer said he did not agree with the policy.

Speaking to ITV News in Brighton, the Labour leader said: “I will vote of course in accordance with the policy that we unveiled on Saturday. We were very clear on Saturday that a GBP10 minimum wage, which is a 12% increase and means GBP2,000 in real money for those who would benefit under that, that is significant, that is our policy.”

The Labour leader denied that he had been pleased about McDonald’s resignation, after speculation that he had been keen to sack him in May’s reshuffle.

But some in the leader’s team believe a confrontation with the left of his party will serve to convince the public Labour has changed.

Starmer said he had spoken to his deputy, Angela Rayner, about the strong language she used at a party fringe meeting on Saturday evening, calling the Conservative government “scum”.

“Angela Rayner and I have different approaches and different styles,” he told SkyNews. Asked if they also had different opinions, he said: “It’s not language I would have used. Angela and I talk every day. Of course we have huge agreements and disagreements.” Rayner refused to apologise after her comments.

Starmer will be joined in the conference hall in Brighton by his wife, after spending part of their family summer holiday hammering out a first draft of his speech.

Many colleagues regard it as a make-or-break moment for his leadership, amid concerns about whether the party has succeeded in gaining a hearing from the electorate – and whether the public knows what Starmer stands for.