Sir Keir Starmer will say on Tuesday that "it will not be business as usual" when Parliament returns next week.
In a speech from Downing Street's rose garden, the prime minister will say that "we can't go on like this anymore" and that his government will do the "hard work" to "root out 14 years of rot" under the previous Conservative administration.
He will also suggest that this summer's riots showed "cracks in our society" but that the "coming together" of communities afterwards showed "what we stand for".
The Conservatives have described the speech as "nothing but performative" and an effort to "distract the public from the promises Starmer made that he never had any intention of keeping".
Sir Keir's address marks a week before Parliament returns from a shortened summer recess and is expected to ramp up attacks on the inheritance left by the Conservatives.
The speech will echo some of the language used by Labour during and after the general election campaign.
Sir Keir will say "the business of politics will resume, but it will not be business as usual" and the government will "get a grip" on the problems faced in the UK.
The PM will warn that "things will get worse" in the UK before they get better as the Labour administration tries to deal with "not just an economic black hole but a societal black hole".
He will say: "And that is why we have to take action and do things differently.
"Part of that is being honest with people - about the choices we face. And how tough this will be.
"Frankly - things will get worse before we get better."
Sir Keir's speech will also suggest that the riots earlier this month showed "the cracks in our society after 14 years of populism and failure".
"The riots didn't just betray the sickness, they revealed the cure, found not in the cynical conflict of populism but in the coming together of a country the morning after" to clear "up their community".
Since coming to power in July, Labour has sought to emphasise the challenges it faces in government and blame Tory ministers for failing to address those problems prior to the election.
The PM's speech on Tuesday will be in front of some 50 members of the public whom he met on the election campaign trail, including small business owners and public servants.
He will promise "no more politics of performance" and to do "the hard work needed to root out 14 years of rot and reverse a decade of decline".
"We'll fix the foundations, protecting taxpayers' money and people's living standards. We'll reform our planning system to build the new homes we need," Sir Keir will say.
"We'll level up workers' rights so people have security, dignity and respect. We'll strengthen our border security. We'll crack down on crime. We'll transform public transport. And we'll give our children the opportunities they need to succeed."
Chancellor Rachel Reeves is widely expected to raise some taxes in her budget on 30 October after accusing the Tories of leaving a £22 billion shortfall in the public finances.
Labour has already been criticised for a move to start means-testing pensioners' winter fuel allowance, restricting payments to only those who receive pension credits or other means-tested benefits.
Reacting to details of the PM's speech, Conservative Party chairman Richard Fuller said: "This is nothing but a performative speech to distract the public from the promises Starmer made that he never had any intention of keeping."
"In fewer than 100 days, the Labour Party has dumped its ambition of public service and become engulfed in sleaze, handed out bumper payouts to its union paymasters with 'no strings' attached and laid the groundwork to harm pensioners and tax working people," the Conservative Party chairman added.
