WASHINGTON — Medicare is wasting more than $8 billion on an experimental program that rewards providers of mediocre health care and is unlikely to produce useful results, federal investigators say.

A report to be issued today by the Government Accountability Office, a nonpartisan investigative arm of Congress, urges the Obama administration to cancel the program, which pays bonuses to health-insurance companies caring for millions of Medicare beneficiaries.

Administration officials refused to cancel the project, saying it could improve care for older Americans.

In the 2010 health-care law, Congress cut Medicare payments to managed-care plans, known as Medicare Advantage, and authorized bonus payments to those that provide high-quality care.