In the most important political speech of his career, Sir Keir will claim “Labour will be back in business”, able to grapple with the big issues facing the country, including recovering from the pandemic and tackling climate change.
Allies of the Labour leader said the speech will show how the party has changed since Mr Corbyn led it to electoral disaster in 2019.
The speech will be the biggest Sir Keir has delivered during a leadership tenure which has coincided with coronavirus restrictions preventing him from addressing large audiences.
He will highlight the fuel crisis and the rising cost of living as evidence that Mr Johnson lacks the competence required for the role of Prime Minister, with the Government “lost in the woods”.
Sir Keir took over from Mr Corbyn after the party suffered its worst result since 1935 as the Tories won a landslide victory in 2019.
In an attempt to convince voters that Labour is a credible government-in-waiting, Sir Keir will say: “Too often in the history of this party our dream of the good society falls foul of the belief that we will not run a strong economy.
“But you don’t get one without the other.
“And under my leadership we are committed to both.
“I can promise you that under my leadership Labour will be back in business.”
A source said the speech will be “noticeably different from what you’ve heard from Labour in recent years”, adding: “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.”
The speech, which Sir Keir began writing during a holiday in Dorset over the summer, draws on his experience of talking to voters who have deserted Labour.
The party believes that voters in Red Wall seats, who had given Mr Johnson the benefit of the doubt, were now concerned about his competence.
A Labour spokesman said: “We know the shine is coming off Boris Johnson, we know that we are in a situation where there are serious questions around the competency of the Government, its ability to deliver.”
Sir Keir’s speech will present Labour as a credible opposition versus a Government which is “strong on slogans and weak on strategy”, the spokesman said.
“I see the Government lost in the woods with two paths beckoning,” Sir Keir will say.
“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.
“That path leads to a future in which a smart government enlists the brilliance of scientific invention to create an economy in which people are healthy and well-educated.
“A contribution society in which everyone has their role to play.”
The country faces big issues, including “how we make our living in a competitive world”, the future of the Union and the relationship with Europe.
“These are big issues. But our politics is so small,” Sir Keir will say.
“So our politics needs to grow to meet the scale of the challenge.”
He will use the speech to pledge that mental health treatment would be available within a month for all who need it under a Labour government.
And he will promise “the most ambitious school improvement plan ever” if Labour wins power, recruiting thousands of teachers and focusing on turning round struggling schools.
The keynote address will close a conference which has seen the resignation of Andy McDonald from the shadow cabinet and a bust-up with unions and the party left over internal rule changes.
But Sir Keir’s allies believe the rows with the Corbynite wing of the party were necessary to have now, to prepare the party for a snap general election if Mr Johnson goes to the country ahead of the scheduled 2024 vote.
“What we will be using the speech to do is to show that these are serious times that require a serious leader, and that is Keir Starmer,” the spokesman said.