A state investigation found that Kaiser Foundation Health Plans wrongly denied patients timely access to mental health care, possibly worsening their conditions.
Kaiser provides health care to more than 6.8 million members in California and is subject to routine surveys by the state Department of Managed Health Care.
In a survey released this week, the department cited several deficiencies in Kaiser's mental health care - which by law must be as easily accessible as physical health care.
In one Kaiser Northern California facility, the average proportion of patients waiting 15 days or more for an initial appointment rose from 7 percent to 60 percent in eight months ending in January 2012.
The report said, "The Plan ... acknowledged the seriousness of this issue."
During the same time period, another Kaiser Northern California facility reported similar results - with an average of 43 percent of new appointments booked more than 15 days out.
According to the state regulations, initial appointments for behavioral health care must be made available in 10 business days, or 14 calendar days.
Kaiser's response blamed low staffing, a slow economy and increased demand due to growing enrollment in the health plan.
The nonprofit said it was implementing new methodology to measure wait times and had established committees to monitor compliance.
Still, the state report said, Kaiser's fix was not enough. "The Department believes ... this deficiency to be serious ... () substantial barriers to accessing care.
"Due to the nature of the violations uncovered, these matters will be immediately referred to the Department's Office of Enforcement," the report concluded.
The state also found that Kaiser described its mental health services to patients in a way that could have discouraged them from seeking care for services that should be covered under law.
Information sheets given to Kaiser patients said the health plan offers "brief, problem solution-focused individual counseling. Research shows many people improve in a single visit.
"In general, we do not begin treatment with individuals whose problems are of such a long-standing nature ... such as chronic mental illness , lifelong personality problems etc.
"We will refer such individuals to an appropriate non-Kaiser facility, although this treatment will not be a Kaiser covered benefit and will not be paid for by Kaiser."
The state report concluded: "These statements are in error because the Plan is required to provide coverage for serious mental illnesses under the same terms and conditions as medical conditions."
Distributing such materials "may have discouraged some enrollees from seeking and accessing medically necessary behavioral health services," the state's report said.
The department plans to conduct a follow-up investigation within the year, it said.
Call The Bee's Cynthia H. Craft, (916) 321-1270. Follow her on Twitter @cynthiahcraft.