Cigna Corp. announced Friday that it is temporarily banned from enrolling new customers in its Medicare Advantage and standalone prescription-drug plans following an October audit from the US Centers for Medicare and Medicaid Services (CMS).

In a letter dated January 21, the CMS reported that “widespread and systemic failures” at the insurer resulted in customers being denied medical coverage and prescriptions they rightly had access to. After customers filed grievances about being inappropriately denied services, Cigna failed to properly address those complaints, the CMS said.

The problems at Cigna, which the agency noted has a “long-standing history of non-compliance,” created a “serious threat to the health and safety of Medicare beneficiaries.” Those beneficiaries are people over 65 and young people with disabilities.

The enrollment freeze will stay in place until the insurer corrects the problems. Cigna told The Wall Street Journal that it was “working to resolve these matters as quickly as possible and is cooperating fully with CMS on its review.”

The sanctions do not affect Cigna's current customers—estimated to be about half a million Medicare Advantage customers and 1.1 million prescription-drug plan customers. But with those plans making up about a quarter of Cigna's revenue, prolonged sanctions could affect a current plan for insurance giant Anthem to buy up Cigna in an approximately $45 billion deal. Federal antitrust regulators are currently reviewing the deal.