Sir Keir Starmer will warn life in the UK is “going to get worse” before it improves in his first major speech as prime minister.

Sir Keir will mark a week before Parliament returns after a shortened summer recess by continuing his attacks on the previous government, saying things are “worse than we ever imagined”.

In his remarks on Tuesday, the prime minister will say he and his ministers “inherited not just an economic black hole but a societal black hole. 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 will say the financial situation is “worse than we ever imagined”, as he repeats Chancellor of the Exchequer, Rachel Reeves’s claims that Rishi Sunak’s Conservatives left a £22bn black hole in this year’s budget.

He will say: “In the first few weeks, we discovered a £22bn black hole in the public finances. And don’t let anyone say that this is performative, or playing politics.

“The OBR (Office for Budget Responsibility) did not know about this. They wrote a letter saying so. They didn’t know – because the last government hid it.”

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The Tories, in power since 2010, had presided over “14 years of populism and failure”, Sir Keir will say, which allowed those taking part in the recent riots to “exploit the cracks in our society”.

One of Labour’s first acts in government was to reduce the proportion of the sentences offenders must spend in prison before being released on parole.

Ministers said it was necessary because the previous government had allowed jails to almost completely run out of space.

Earlier this week, the government triggered Operation Early Dawn – meaning defendants could be held in police cells for longer until prison space becomes available.

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In his speech, Sir Keir will say: “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.”

Arguing that change will not happen “overnight”, the prime minister is also expected to say Labour has achieved “more in seven weeks than the last government did in seven years”, including setting up a National Wealth Fund, changing planning policy to build more homes and ending public sector strikes.

Conservative Party chairman, Richard Fuller MP, said: “Just two months in and Keir Starmer has taken winter fuel payments off 10 million pensioners, showered billions of taxpayers’ money on his union paymasters and is now engulfed in a cronyism scandal after parachuting donors and supporters in to top taxpayer-funded jobs.

“The soft touch Labour chancellor is squandering money whilst fabricating a financial black hole in an attempt to con the public into accepting tax rises, and literally leaving pensioners in the cold.

“The prime minister really should tell his chancellor to reverse course or step in himself to reverse her decision.”

Sir Keir’s speech comes ahead of a potentially tough period for the government as it prepares its first budget, due on 30 October.