Extra and immediate cuts to County Hall's already severe budget plan would need to be made... if its One Angel Square headquarters is not sold come April.
The warning comes in chief finance officer Mark McLaughlin's no-holds-barred comment on councillors' final budget plans.
He said if a buyer for the new £52 million building were not to be found and the deal not pushed this financial year "rapid and difficult reductions in non-statutory services would be unavoidable."
Mr McLaughlin added that even if the sale took place and the price could not bridge the gap in funds "the only recourse at that point will be the elimination of non-statutory spending and this would almost certainly involve, again, a further set of difficult decisions."
The chief financial officer - who made the national news two weeks ago with his decision to halt all new council spending - said further sales of county council assets will be needed but stressed this was not a sustainable approach and needs to be married to "iron discipline in decision making and delivery". Mr McLaughlin said the softening of some of the original plans, as reported by the Chron last week, are not in line with that policy.
Stressing the need to be "realistic", he said financial discipline must be led by councillors and "this should begin with the acceptance of the maximum number of realistic savings options, including those which have proved controversial or unpopular during the consultation process during the draft budget."
Mr McLaughlin continued: "Reversing the consulted-upon savings options on libraries, bus subsidies and trading standards to the value of £1.56m as proposed in this budget may not be sustainable even in the short term."
He said that senior councillors had for years set budgets that were unrealistic and led to them falling further and further behind in their quest to balance the books.
Elsewhere in the report - which goes before full council on Thursday, February 22 - the financial officer says the council cannot rest all hopes on a ministers coming up with some "new system of council funding", pointing out that "similar promises have been made and broken in the past".
Financial salvation is most likely to be found in creating one or more unitary councils in an effort to save money, Mr McLaughlin said, which district and borough councils have been reluctant to consider as it would lead to their abolition.
He said: "Achieving this will require leadership,partnership and a focus on the needs of the residents of the county and the suppression of the vested producer interests, including those of local Government members and officers."