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.