Police Scotland is likely to overspend its budget for the current year by £17.5m, according to the body which oversees the force.
The Scottish Police Authority (SPA) said the main reason was staff costs, but added these were being "managed down" with strict workforce planning.
Its report concedes more work is needed to bring spending in line with budget.
The Scottish government had pledged to keep police numbers at more than 17,200 between 2007 and April of this year.
The pledge was then dropped, although the Scottish government said in its draft budget that it would keep the commitment in the current financial year, while "working with the SPA to consider the implications of changing demands on Scottish policing."
Ministers also have a strict policy of no compulsory redundancies for civilian staff.
The warning from the Scottish Police Authority comes two weeks after the Scottish Police Federation (SPF), which represents rank and file officers, said the force was using patrol cars held together with duct tape and cable ties.
Calum Steele, of the SPF, also said interview suites for sex assault victims were damp and had mouldy carpets.
Mr Steele blamed decades of under-investment which had left a "crumbling police estate".
Speaking at a fringe meeting at the SNP conference in Glasgow on 14 October, Mr Steele showed pictures that he said came from inside police buildings and a photo of a car being used by officers.
"We have a crisis, a genuine crisis, coming round the corner as far as our facilities are concerned," he told the meeting.
The SPA said Police Scotland's capital budget was forecast to underspend by £9.6m against its budget for 2016-17. The revenue budget has a predicted overspend of £27.1m.
Combined, this brings the total forecast overspend to £17.5m.
The SPA's report concludes: "At present the organisation is facing an overspend against budget for the year. Further work is now required to manage this forecast downwards over the coming months to bring expenditure back in to line with the budget."
Scottish Conservative justice spokesman Douglas Ross said: "This is a startling increase in overspend, and shows that police budgets are starting to spiral out of control.
"It is no secret that Police Scotland are facing budgetary pressures, and there are serious questions about whether they have the resources needed to carry out their job.
"The SNP need to explain why this increase in overspend is happening, and reassure the public that it will not have a knock on effect to frontline policing."
And Claire Baker of Scottish Labour said SNP election pledges to protect police budgets had been "nothing more than a cheap sound bite".
He added: "The public are seeing the reality in their own communities, as underfunding by the SNP is leading to local stations closing and officer numbers at their lowest level since 2010. This can't go on."
Responding to criticism about police budgets earlier in October, the Scottish government said: "In terms of funding, the Scottish government is committed to protecting the police revenue budget in real terms for the entirety of this parliament, delivering an additional £100m of investment over the next five years, in addition to £55m of reform funding in 2016-17.
"It is for the Scottish Police Authority and Police Scotland to determine the best possible use of the budget according to national and local priorities."