Sir Keir Starmer is set to say that Parliament will not be "business as usual" when it returns from a shortened summer recess next week.
The Prime Minister is set to make a speech from Downing Street's rose garden as he pledges to "get a grip" on problems faced in the UK.
He will say "the business of politics will resume, but it will not be business as usual", before warning that "things will get worse" as his party deals with "not just an economic black hole but a societal black hole".
He'll add: "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."
However, the Conservatives have described the speech as "nothing but performative".
Party chairman Richard Fuller said: "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 speech by the Prime Minister - which will be made on Tuesday - has been startegically planned in Downing Street's rose garden as a "symbol of the rot at the heart of (the Conservative) government".
During the pandemic, Dominic Cummings gave a press conference in the garden defending his decision to break lockdown rules and travel more than 250 miles.
Sir Keir 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".