The Prime Minister is set to warn that Britain will ‘get worse before it gets better’ in his first major speech since taking office.

Sir Keir Starmer is expected to use the speech on Tuesday to ramp up attacks on the inheritance left by the Conservatives.

He will say: “We have 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.”

Since coming to power in July, Labour has sought to emphasise the challenges it faces in government and pile blame on the Conservatives for failing to address those problems prior to the election. Rachel Reeves, the Chancellor, began that theme in her statement before the summer recess in a statement accusing the Tories of leaving a £22 billion black hole in this year’s budget.

Sir Keir is expected to develop that line of attack further on Tuesday, saying things are “worse than we ever imagined”.

He will say: “In the first few weeks, we discovered a £22 billion black hole in the public finances. And don’t let anyone say that this is performative, or playing politics. The OBR did not know about this. They wrote a letter saying so. They didn’t know – because the last government hid it.”

But he will also address the condition of the wider public sector, saying those taking part in the recent riots had been able to exploit “the cracks in our society after 14 years of populism and failure”, including a lack of prison places.

One of Labour’s first acts in government was to reduce the proportion of their sentences offenders must spend in prison before being released on parole, arguing the previous government had allowed jails to almost completely run out of space.

He 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.

Sir Keir’s speech comes ahead of a potentially tough period for the Government as it prepares its first budget, due on October 30. Ms Reeves faces the challenge of delivering on Labour’s promises on taxation and spending amid what she has already described as a worse fiscal situation than she was expecting, with some tax rises looking increasingly likely.

The Government also faces the prospect of another rebellion over its plans to restrict the winter fuel payment to only the poorest pensioners, especially in light of Friday’s announcement of an increase in the energy price cap.

The Chancellor has argued this is necessary to help bridge the £22 billion gap in this year’s budget, but opposition parties and some Labour backbenchers have warned about the impact on pensioners with incomes just above the limit for claiming pension credit.

But with its large majority, the Government is unlikely to see a defeat should the policy come to a vote in the Commons.

Conservative Party chairman Richard Fuller 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 into top taxpayer-funded jobs.

“The soft touch Labour Chancellor is squandering money whilst fabricating a financial black hole in an attempt to com 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.”