WASHINGTON — Federal officials say they have repeatedly criticized, and in many cases penalized, Medicare health plans for serious deficiencies, including the improper rejection of claims for medical services and unjustified limits on coverage of prescription drugs.

The findings, cataloged in dozens of federal audit reports, come as millions of older Americans prepare to sign up for private health plans and prescription drug plans in Medicare’s annual open enrollment period, which will begin on Wednesday and continue through Dec. 7.

About 16 million people, accounting for 30 percent of the 54 million beneficiaries, are in private Medicare Advantage plans, which provide a full range of health care services under contract with the government. An additional 23 million people are in prescription drug plans, which cover only medications.

Federal officials expressed frustration that they were seeing the same kinds of deficiencies year after year. In a memorandum to health plans, Gerard J. Mulcahy, the Medicare official in charge of oversight and enforcement, listed common “areas of noncompliance” identified in program audits.