Federal workers who depend on government-backed health insurance to help pay a major portion of their yearly multimillion-dollar bills for psychiatric and mental health treatment are already hearing rumors that the program is to be cut back next year.

Each year about this time newspaper, congressional and government offices get calls from worried civil servants. They fear that secret negotiations between the Office of Personnel Management and the many health insurance carriers in the program will result in mental health benefit cutbacks. More than 8 million federal employees and their families are covered by the health insurance program, the largest of its kind in the world.

Government officials suspect some of the calls are inspired by psychiatrists and/or mental health professionals worried about their patients -- and benefits cutbacks. Some of the callers say they are doing it because insiders at OPM and on Capitol Hill tip them that top people want to see psychiatric benefits reduced to cut costs, or offer better benefits for other medical services.

Blue Cross-Blue Shield is the biggest firm in the federal health benefits programs. It covers approximately 5.5 million workers and family members, and has one of the best (and one of the few) policies that pay mental health-related benefits.

In 1978 (latest figures the OPM has) the Blue Cross-Blue Shield plan, covering 5.5 million workers and family members, paid out $119.6 million in mental health related claims and benefits to 72,800 insured employes or their spouses or children. That is just over 1 percent of the people covered by the Blues' government health plan which, overall that year, paid out $1,450 in benefits.

There is a conflict between federal workers who would like to see other benefits broadened, the dental vs. mental argument. Although the number of federal workers and family members seeing psychiatrists is smaller that the number who visit the dentist, more than 8 percent of all health claims go for mental health services. Dental insurance in government is just beginning to get off the ground.

An official who asked not to be quoted ("not on something like this") said government insurance negotiators are told to get the best health insurance coverage for the least money, and that "they have to make a lot of hard choices." He said the mental health costs, which can be very, very high, are "extremely important" to a "relatively small number of people . . . including some members of the medical community." On the other hand, he said, there is a growing demand for improved dental health protection (it was one of the major projects former Rep. Gladys N. Spellman had planned for this year) that would, at least numerically, benefit more people.

The dental vs. mental issue could become a hot one this summer when new rates and benefit packages are announced.