Rachel Reeves’ promise of honesty is a risky move.

Rachel Reeves will make a statement to parliament on Monday promising to be “honest” about the scale of the fiscal challenge facing the new Labour government.

It’s not about new policy or new borrowing expectations.

Instead, Treasury officials have begun scrutinising ministerial plans to uncover some of the unexpected costs and implicit cuts to public services, under spending plans inherited from the previous government.

Publishing a comprehensive report on public spending pressures is risky. It will be a crucial test for the new chancellor.

And when she stands up and tells MPs that she has discovered billions of pounds of unexpected spending pressure, many of them will ask questions.

Were these pressures really unexpected? And were they saved from tearing up the files of the previous government?

The first task for the new chancellor is to gain credibility with investors – and with voters. Everything the finance minister achieves on the economic front is made easier if he has that trust.

But the past few years have proven that credibility is hard to gain and much easier to lose.

Losing it makes even simple politics difficult.

Reeves has been at pains to prevent any future government from sidelining the UK’s independent forecaster from assessing its economic plans, as former Prime Minister Liz Truss did in her short-term mini-budget.

In fact, she is working on legislation that would give the Office of Budget Responsibility (OBR) powers to rule on any major tax or spending announcements.

The Office for Budget Responsibility will not be involved in Monday’s process, although we may hear from them after the event.

In previous reports, the government has suggested that it has regularly miscounted borrowing in official forecasts because it underestimated government spending.

Last year, the agency specifically cited its legal requirement to link its forecasts to “stated government policies” even when those policies are seen as unrealistic.

For example, the Office for Budget Responsibility said in March that real per capita spending in the UK was likely to be 8% lower in 2027 than originally forecast in the latest spending review, which included government departments’ public spending plans.

The question on Monday is whether Reeves has detailed evidence of what this means in practice.

Home Secretary Evatt Cooper has already said the now-scrapped Conservative plan for Rwanda cost £700m. In areas not protected from spending cuts, the civil service was supposed to be working out scenarios for the real-world impact of cuts on prisons, courts, universities, higher education colleges and councils, including adult and children’s social care.

The new government seems to be suggesting that the huge spending required on essential public services means there is little room for choice. For example, local councils spend the bulk of their budgets on services they have a legal obligation to provide.

It is no surprise that spending in some areas is under severe pressure.

But were the public sector payroll adjustment recommendations, which are about 3% higher than current spending plans, that surprising? Regardless, it is up to the government to decide whether to accept and fund them.

In almost every parliament (except the last one) new governments have chosen to raise taxes in their first budgets. Monday’s plan looks like it’s part of a process leading to tax increases in the fall.

The government has already ruled out raising tax rates, which account for 75% of revenues.

But the new chancellor will need her officials to find some evidence in their ministry’s official documents, or have to explain why all this was not made clear to voters before the election.