Labour have accused the Conservatives of “covering up” the scale of problems in the public sector, creating a multi-billion pound gap in department budgets.

Environment Secretary Steve Reed told the BBC’s Sunday with Laura Kuenssberg programme incoming ministers had found “catastrophic” issues in the justice system, Rwanda scheme and flood defences after taking office.

His comments came ahead of a speech on Monday where Chancellor Rachel Reeves is expected to outline a £20bn a year black hole in the public budget she says has discovered.

The Conservatives have accused Labour of “lying” and said the Office for Budget Responsibility (OBR) regularly publishes updates on public finances.

Mr Reed said Labour had found problems “we could not have known during the election” because the Conservatives had not released information and in “some cases deliberately covered it up”.

“We are finding additional pressures in year that nobody knew about and the government had not disclosed,” he said.

He accused Rishi Sunak of hiding warnings by civil servants of a “critical failure” in the prison system that meant “by August there would be no prison places left”.

Instead of taking action, Mr Sunak “called a general election” and “covered up that information until after the general election”, the environment secretary added.

While the government publishes weekly figures showing prison numbers, Mr Reed said these were published “in arrears” so were not up to date.

Labour has announced plans to release thousands of prisoners early from the start of September to prevent a “total collapse” of the prison system and a “total breakdown of law and order”.

Mr Reed also argued the true scale of the cost of the Rwanda scheme was not revealed until “Yvette Cooper is appointed home secretary, and goes into the department”.

Last week Ms Cooper said the Conservative’s plan to remove some asylum seekers to Rwanda had cost taxpayers £700m – nearly double the price tag previously in the public domain. The new figure includes additional costs such as civil service salaries and detention costs.

Mr Reed said of his own department: “We are finding that the condition of flood defences is far worse than we were led to believe.”

He added:”That therefore puts more pressure on the public finances if we want to sort that out.”

He said: “We want to move away from this government of secrecy to a government of openness and transparency”.

Pushed on whether Labour were revealing these figures now in order to lay the foundations for tax rises, Mr Reed repeated Labour’s election promise “not to increase taxes on working people”.

But on Sky news Mr Reed did not rule out increases in capital gains, inheritance tax and taxes on pensions.

He said: “The core of what we want to do is not to increase taxation, it is to grow the economy because that way we can get the income we need without recourse to taxation.”

Responding to Mr Reed’s comments, backbench Tory MP Alicia Kerns said: “It is nonsense to say they don’t know how the sums add up.

“They have had the OBR and they have had all the detail.”

On Monday, the chancellor will reveal the results of a Treasury audit on the state of public finances.

Ms Reeves previously told the BBC she would “fix the mess we inherited” but would not confirm speculation that the gap in the public finances stood at more than £20bn per year.

The statement is widely expected to preview a mix of new taxes and budget reductions to plug shortfalls in the nation’s finances.

A government spokesperson said the audit had shown “the previous government made significant funding commitments for this financial year without knowing where the money would come from”.

The spokesperson said: “The assessment will show that Britain is broke and broken – revealing the mess that populist politics has made of the economy and public services.”

The Conservatives accused the new government of “peddling nonsense”.

The Tory shadow chancellor Jeremy Hunt said: “Since we established the OBR in 2010, the books have been wide open and what they show is a healthy, growing economy – not the fiction Labour is now peddling which is widely rejected by independent commentators.

“Their motive is clear: having promised not to raise taxes 50 times before the election they now need a pretext but trying to scam the British people so soon after being elected is a high-risk strategy doomed to fail.”