Sir Keir Starmer will warn the British public that things will get worse before they get better in his first keynote speech since walking into Downing Street.
Speaking on Tuesday, the prime minister will say the government has also taken on a “societal black hole”.
It will come after chancellor Rachel Reeves claimed the government inherited a £22bn black hole in the public finances from the previous Conservative administration last month.
“Things are worse than we ever imagined”, he will warn, adding: “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.”
The warnings come as the government prepares for its first Budget, to be unveiled on 30 October, which will force the Treasury to take tough decisions on spending and taxation.
Promising to make “unpopular decisions now if it’s the right thing for the country”, Sir Keir will also accuse the Conservatives of not being honest about the state they left the country in.
He will say: “They haven’t recognised what they’ve cost the country and they haven’t apologised for what they’ve cost you.”
The prime minister is also expected to promise to “fix the foundations of this country”, warning “change will not happen overnight”.
He will say: “When there is rot deep in the heart of a structure, you can’t just cover it up. You can’t tinker with it or rely on quick fixes. You have to overhaul the entire thing. Tackle it at the root. Even if it’s harder work and takes more time.
“Because otherwise what happens? The rot returns. In all the same places. And it spreads. Worse than before. You know that – and I know that. That’s why this project has always been about fixing the foundations of this country.”
He will also argue that “14 years of populism and failure” under the Conservatives made responding to this month’s riots much harder than in 2011, when he led the Crown Prosecution Service, claiming that violent thugs exploited “the cracks in our society”.
Just weeks after receiving the keys to Downing Street, Sir Keir revealed emergency measures to free prisoners 40 per cent of the way into their sentence, saying the previous government had allowed jails to almost completely run out of space.
In 2011, rioting and looting led to more than £200m worth of damage and more than 3,000 arrests.
Sir Keir will say: “When I think back to that time, I see just how far we’ve fallen. Because responding to those riots was hard, but dealing with the riots this summer was much harder.
“Not having enough prison spaces is about as fundamental a failure as you can get. And those people throwing rocks, torching cars, making threats, they didn’t just know the system was broken. They were betting on it. They were gaming it.
“They saw the cracks in our society after 14 years of populism and failure, and they exploited them. That’s what we have inherited.”