The United States, once the leader in making family planning information and services available to women of child-bearing age, has plummeted to seventh place among developed nations, according to a report released yesterday by the Population Crisis Committee.

The PCC, a nonprofit organization advocating worldwide family planning, found that the United Kingdom "has the best family planning programs in the world."

The "World Assessment," PCC spokesmen said, is the first rating of family planning in 110 countries, developed and undeveloped. It found that about half the world's population has only fair, poor or very poor availability of family planning services.

According to Dr. J. Joseph Speidel, vice president of PCC and technical editor of "World Assessment," the past few years have seen a drop in support for worldwide family planning with the result that there is "an epidemic of illegal abortions" throughout the world. The PCC estimates that in the developing countries, some 250 million women need and want contraceptive services and "either do not have access to services or lack the education and information needed to use them."

The PCC actually conducted two surveys, one on 15 highly industrialized countries and a second on 95 Third World countries. The ratings were based on availability of safe, legal abortion, oral contraceptives, intrauterine devices, female and male sterilization, other methods including condoms, diaphragms, caps, sponges and spermicides. Ratings were also made according to the availability of services to the poor, to minors, sex education in schools and contraceptive information and advertising.

The top-rated developed countries behind Britain were West Germany, Australia, Canada, France, Italy, the United States, Czechoslovakia, East Germany and Yugoslavia. Countries with lower ratings were Poland, Spain, Japan and the U.S.S.R. However, the only developed country to fall into a category deemed "poor" is Romania, where the government is encouraging a higher birth rate.

Among the developing nations, Taiwan, Singapore, South Korea, China and Hong Kong are rated as "excellent." In fact, the PCC notes that China, with its rigorous population restrictions, has "skewed the averages" because its birth rate has dropped 55 percent over the past 15 years.