The federal government should stop turning its back on childless couples who want babies by supporting test-tube conception after years of "silence and inattention" and a "de facto" prohibition of such research, one of the nation's leading authorities on the ethics of medical care said yesterday.

First, said Dr. LeRoy Walters, director of the Center for Bioethics, part of Georgetown University's Kennedy Institute of Ethics, the Department of Health and Human Services should quit ignoring a 2 1/2-year-old recommendation calling both laboratory research in test-tube conception and human use of the method ethically "acceptable."

Second, he said, the department should end its eight-year ban on federal funds for any such efforts.

This ban has inhibited most medical centers that receive any federal funds from beginning test-tube efforts, even with private funds, lest federal officials call them "unethical." Such a label, researchers fear, could lose them other federal funding, and open the way to malpractice suits should any test-tube efforts go wrong.

Test-tube conception means union of human ova (female egg cells) and male sperm in the laboratory and subsequent implantation of the developing embryo into the mother. In recent months, the method has suddenly begun to achieve startling new success, with at least 21 such births in three countries.

Walters' statement is the first call for federal support of test-tube conception since the birth of the first American-born test-tube baby in Norfolk six days ago. Walters is a leading voice at the Kennedy Institute, one of the leading centers of both scholarly and religious examination of delicate medical issues.

There has been some religious opposition to test-tube conception, as "violating the laws of nature" and leading to future "genetic engineering." Some of the opposition has come from Catholic leaders, but there is no firm church position on the issue; Georgetown is a Catholic university.

Walters was backed in his recommendations by Dr. John Biggers, professsor of physiology and a specialist in human reproductive biology at Harvard University.

Walters and Biggers spoke at a news conference on the opening day of the annual meeting here of the American Association for the Advancement of Science. Both will present their views at a formal symposium today, one of 160 in the next five days. The AAAS has 136,000 dues-paying members, as well as 245 affiliated societies with millions of members.

The prohibition on use of federal funds for any research involving human fetuses at any stage of development was imposed by the then-Department of Health, Education, and Welfare in 1975. It has been modified to permit some highly limited studies of aborted fetuses.

The government has supported research in "in vitro" or test-tube fertilization only in animals. The federal hesitation, Biggers said, has "greatly retarded" human efforts in this country, despite the birth in Norfolk last week.

In 1977 and 1978, an HEW Ethics Advisory Board considered the subject for nine months and listened to friends and foes of the in vitro fertilization as well as many students of human and medical ethics. In May, 1978, the board told then-HEW Secretary Joseph A. Califano Jr. it found no ethical objection to using laboratory conception to help infertile married couples have babies.

Califano and every HEW and HSS secretary since has declined to act or avoided the subject.

At the least, said Walters and Biggers, the government should begin a "close watch" on the results of American and other test-tube conceptions. No one is keeping a systematic record of all such records and their consequences, including the important matter of the current and future health of the children born to such unions.

So far, Biggers said, the procedure seems to be as safe as natural fertilization. Only one baby has been born with a birth defect, a boy in Australia with a congenital heart defect that will have to be repaired by surgery when he is older.

About 600,000 American couples could potentially be helped by test-tube baby techniques.

Walters also said health insurers should start paying for the costs of test-tube fertilization and birth, which may run from $4,000 to $8,000 at various centers. Otherwise, he said, the procedure will be only one for "the rich."

Before Long, Walters predicted, society may have to deal with still another startling advance: the probable ability to alter human genes, either to prevent diseases or to impart some special qualities, like improved brain power or memory.

Plainly, he said, it should be considered ethical to do so to prevent such devastating genetically transmitted ailments as Tay-Sachs disease and sickle cell anemia.

But also, he said, it should be considered acceptable in some cases to "enhance human capabilities." At the least, he said, "we should not bar the door entirely" to such uses though we should "reflect carefully and well in advance" to try to prevent misuses by some future genetic engineers.