Services to taxpayers are likely to drop to their worst levels since 2001, when the Internal Revenue Service first started measuring its performance, the agency’s taxpayer advocate said in her annual report released on Wednesday.

Five years of budget-cutting have “brought about a devastating erosion of taxpayer service, harming taxpayers individually and collectively,” wrote Nina E. Olson, who leads the Taxpayer Advocate Service, an independent office within the I.R.S.

Ms. Olson warned that in this fiscal year — as filing requirements mandated by the Affordable Care Act go into effect — the agency might end up answering as few as 43 percent of the more than 100 million phone calls it expects to receive from taxpayers.

Callers who manage to get through are expected to be on hold for an average of 30 minutes. That is a decline from the 61 percent of calls answered and average waiting time of 18 minutes in the 2013 fiscal year.