An audit of public spending pressures will see claims of a “black hole” worth tens of billions of pounds by the new government.

Chancellor Rachel Reeves said she would give a statement to Parliament on Monday showing “honesty” about the scale of the challenge faced by the new Labour government.

She vowed to “fix the mess we inherited” but would not confirm speculation that the “black hole” stood at more than £20bn per year.

BBC News has contacted the Conservative Party for a response.

The new government will suggest its predecessor left various crucial public services unfunded in areas from public pay to prisons.

“On Monday, the British public are finally going to see the true scale of the damage the Conservatives have done to the public finances,” a Labour source said.

“They spent taxpayers’ money like no tomorrow because they knew someone else would have to pick up the bill. It now falls to Labour to fix the foundations of our economy and that work has already begun.”

On Monday, Ms Reeves will also set out the government’s response to the public sector pay recommendations, which are about 3% higher than in current spending plans.

The chancellor is likely to accept some above-inflation pay settlements for public sector workers after being warned by independent pay bodies about recruitment and retention challenges, and calculating that the public is keen to draw a line under months of rolling strikes.

Cabinet ministers have spent the week saying a thorough look at their departmental books has revealed more “severe” problems than previously anticipated.

The opposition say this is an elaborate effort to butter up the public for some tax rises at the Budget in the autumn.

The “black hole” arises because the government says it must spend extra money to keep public services functioning.

However, governments draw up their own rules on how much they should borrow to fund public services like the NHS and how the country’s debt is managed.

Paul Johnson, director of the Institute for Fiscal Studies think tank, said it was not credible that the government had now looked at the books and found problems to be more severe than expected, given how many organisations had pointed out that most public services were now performing “considerably worse” than they were pre-Covid.

“The choice, as ever, is do you want the public services to be as good as they are, better than they are, or are you willing to see them get worse?” he told BBC Radio 4’s Today programme.

The extra money the government needs to find to meet self-imposed targets around debt in the future has been labelled the “black hole”.

The former chancellor, Jeremy Hunt, may have instead suggested not spending these sums.

Mr Hunt acknowledged to the BBC this week that he would not have been able to fund immediate tax cuts, had the Conservatives won the election.

Some economists suggested during the election that many of these spending pressures, and the possibility taxes would need to be raised to fund them, were obvious and should have been made clear to the voting public.

While the BBC understands there will be no tax policy announcements on Monday, the implication of the audit is that the Treasury will spend the summer trying to find extra savings, or extra taxation revenue, to fill this “black hole”.

A more optimistic economic outlook from the independent forecaster the Office for Budget Responsibility would also help improve some tricky trade-offs.