One of every three persons involuntarily committed to Virginia psychiatric hospitals--or about 2,000 persons a year--may be confined illegally because there is insufficient evidence to justify their detention, a survey by the state Department of Mental Health and Retardation concluded.

A department report, which estimates that more than 6,000 people are involuntarily committed each year to the state's public hospitals, has buttressed arguments by civil liberties groups that state courts routinely are depriving patients of their constitutional rights through commitment proceedings that often are hastily called and informally conducted.

"The situation in Virginia is scandalous," said Leonard S. Rubenstein, president of the state chapter of the American Civil Liberties Union and a lawyer in mental health. "People don't get adequate representation. They don't get adequate evaluations."

Top state mental health officials have played down the findings, attributing the high number of what the report calls "unsubstantiated" involuntary commitments to a lack of community-based programs, such as group homes and emergency shelters, to treat the mentally ill.

"In some cases, it's just a miscalculation in making a decision to commit somebody," said Donald Biskin, an assistant department commissioner whose office prepared the report. "But it's getting better . . . It's something that we've known about and we're trying to work on."

Although the report has not been distributed by the state mental health department, a number of mental health experts have challenged its conclusions.

"They don't know what's going on," said Tom Geib, chief of Arlington County's Mental Health Services Unit. "They judges are not committing people who are saying, 'I think I'm going to commit suicide.' They're committing people who have made an overt act."

Yet a survey of all 287 involuntary admissions to Virginia psychiatric hospitals during a two-week period in April 1982 showed there was insufficient evidence to justify confinement in one-third of the cases, according to observations and examinations conducted by staff psychologists and social workers. That survey formed the basis for the report's conclusions.

In nearly a quarter of the cases, patients displayed no violent or anti-social behavior and even showed evidence of having "fairly good social skills," being "alert and well-oriented" and displaying "a fair degree of self-initiative," the report said.

But some legislators and mental health critics said the report's findings suggest that unjustified commitments are far more frequent than earlier believed. In a sketch of case histories that typify the problem, the report cites the example of a 37-year-old man with a long history of mental illness who, after being rejected for welfare benefits, moves into his parents' home and spends most of his time sleeping in his room.

Frustrated by their son's listlessness, the parents have him involuntarily committed to a mental hospital, even though he has not shown any signs of dangerous behavior or evidence of being worse off than when he was last treated at the same hospital.

"I was appalled when I got this," said Del. Warren G. Stambaugh (D-Arlington), who was chairman of a joint legislative subcommittee on involuntary commitments that requested the report and is proposing a controversial overhaul of the state's commitment laws.

"I started out thinking that the commitment process was a sham . . . ," Stambaugh said. "But it never occurred to me that the procedures were that bad that they would lead to this."

Interestingly, many critics say, Virginia's commitment law is relatively protective of patients' rights compared to other states. Under a 1974 Virginia statute, a person may be involuntarily committed upon the petition of "any responsible person" (usually a parent or relative) if a judge finds that the person poses "an imminent danger to himself or others" or is "substantially unable to care for himself." The judge must also find the person is unsuited for a "less restrictive alternative" to confinement in a hospital.

Arlington's Geib questioned the accuracy of the report's observations of patients confined in a hospital after commitment. Though a genuinely psychotic episode may have triggered the commitment, "an individual can stabilize fairly quickly once he is hospitalized," Geib said.

Dr. Allen Gouse, who wrote the report, said that the evaluations by hospital staff were completed within three to five days of a patient's commitment and were designed to reflect the patient's functional level "upon admission."

The report's findings were released amid debate over a controversial bill proposed by Stambaugh that would rewrite state law governing involuntary commitments. The proposal is billed as a reform of the 1974 statute and addresses some of the problems that critics say have contributed to excessive commitments.

For example, because critics said commitment proceedings are often called too quickly, the bill would stretch the time that a person can be detained in protective custody from two days to three days. That would allow more time for medical treatment before a judge had to rule on a commitment petition, Stambaugh said.

But the measure also adopts language contained in a model bill proposed by the American Psychiatric Association that, critics say, would make the problem even worse, expanding the powers of medical doctors and weakening the legal rights of the mentally ill.

Most dramatically, the proposal would allow a court to order commitment if it finds the person "is likely to suffer substantial mental or emotional deterioration"--a standard that critics say is so subjective and vague it would substantially increase the number of commitments.

"I may have substantially deteriorated since yesterday, but I don't think I'm commitable," said Virginia ACLU president Rubenstein at a hearing on the bill this week. "This language would fill up our institutions with people who don't belong in them."

Stambaugh has acknowledged that the proposed language could create problems. "I must admit this one gives me great pause," he said.