The Food and Drug Administration has approved the marketing of a contraceptive sponge, a new birth control device for women that can be purchased without a prescription and that allows more "sexual spontaneity" than do some other products, company officials said.

The manufacturer, VLI Corp. of Costa Mesa, Calif, said that the one-size-fits-all, disposable polyurethane sponge, soaked with a commonly used sperm-killing chemical, is expected to be available nationwide in early fall. Under the name "Today," the sponges will be sold for about $1 each.

"The testing of the sponge in nearly 1,600 women since 1979 has shown it to be about 85-percent effective in preventing pregnancy. This effectiveness rate is in the same range as other vaginal contraceptives such as the diaphragm or spermicide alone," said FDA official Faye Peterson.

Diaphragms, however, require fitting in a physician's office, and non-prescription products such as foams and gels require insertion immediately before intercourse. The new sponge would provide protection up to 24 hours after insertion, and must be discarded on removal.

Experts say the sponge is likely to make a large dent in the sales of other vaginal birth control products because of its convenience and safety, but will be less appealing to women seeking the most effective means of pregnancy prevention.

It will appeal to all the woman who are "back to basics, people who have been frightened away from the pill and the IUD intrauterine device ," as well as "teen-age girls, mainly because of the ease and privacy connected with it," said Dr. D.J. Patanelli, of the National Institute of Child Health and Human Development.

The institute spent nearly $700,000 in U.S. studies of the sponge, with additional funding for international research from the U.S. Agency for International Development.

"The sponge is an important new method of vaginal contraception. Its single most important attribute is that it can be worn for 24 hours, allowing several acts of coitus without having to repeat its use," said Dr. Louise Tyrer, of Planned Parenthood Federation of America. "But because of its relatively high failure rate, a lot of people will not consider it acceptable," she said.

The 15-percent failure rate of the sponge means that if 100 women used it for a year, 15 would become pregnant, said Dr. Solomon Sobel, head of the FDA division that approved its use. In comparison, the failure rate of the birth control pill is less than 2 percent and, of the IUD, less than 5 percent.

In its approval letter, the FDA said the sponge could not be advertised as "highly effective." The agency said that the company could indicate that tests have found "no evidence of a significant health risk associated with the use of this product," but noted that the studies were not large enough to resolve the question of whether it increases the risk of Toxic Shock Syndrome.

The rare but life-threatening condition has been associated with super-absorbent tampons, particularly in young women. Sobel said that the agency had altered the labeling to say that "use of the vaginal contraceptive sponge during the menstrual period is not recommended."

While the sponge caused few side effects, 3.6 percent of the women tested stopped using it because of irritation, rash or allergic reactions, the FDA said.

Several experts emphasized that the sponge is far from a breakthrough in contraceptive technology. More than 3,000 years ago the Egyptians soaked sea sponges in citrus juices to prevent pregnancy, Patanelli said.