Affiliates of UnitedHealth Group Inc., the nation’s largest health insurer, were hit with a class action lawsuit Tuesday claiming they are illegally underpaying reimbursements for mental health services.

According to the complaint filed in the U.S. District Court for the Northern District of California, United HealthCare Insurance Co. (UHIC) and United Behavioral Health imposed arbitrary reimbursement penalties for certain psychotherapy services provided by psychologists and master’s-level counselors rather than doctors.