The Reagan administration cannot require family-planning clinics to notify parents when their daughters receive contraceptives, a U.S. Court of Appeals panel said yesterday.

The panel, upholding a February lower court ruling that barred such a requirement from taking effect, said that Congress clearly believed "confidentiality was a crucial factor in . . . stemming the epidemic increase in teen-age pregnancies."

The three-judge panel said the administration's proposed regulations are "unlawful" and would undermine the intent of Congress in passing legislation in 1970 to provide nationwide comprehensive family-planning services.

The panel rejected administration arguments that the parental notification rule was justified by a 1981 amendment requiring federally funded family-planning agencies to "encourage family participation" in their services.

The bitterly contested regulation would have required some 5,000 family-planning clinics to notify parents within 10 days after a girl under 18 received prescription contraceptives such as a diaphragm, birth control pills or an intra-uterine device.

Civil liberties and family-planning groups argued that the rule would scare off as many as one-third of the more than 500,000 teen-agers a year who use the clinics and would result in 33,000 teen-agers nationwide becoming pregnant.

But administration officials and others who supported the rule argued that there was no good data to support those conclusions. They insisted that parents have a right to know what their daughters are doing and that the government should not provide contraceptives secretly.

President Reagan recently called the "fight against parental notification . . . only one example of many attempts to water down traditional values . . . of American democracy."

A White House spokesman yesterday declined comment on the ruling, saying officials had not had a chance to study it carefully.

But the Justice Department said in a statement that it disagrees with the opinion and is "anxious to see what the 2nd U.S. Circuit Court of Appeals opinion is in a similar case in New York," where a federal judge also struck down the same regulation.

"The Justice Department and the Department of Health and Human Services are considering available options in light of today's opinion," the statement said, "including the ones Judge Robert H. Bork may have left open in his opinion."

Bork, in a partial dissent from the majority opinion written by Senior Circuit Judge J. Skelly Wright, said that while he agreed with "much of the majority's opinion," he did not believe that Congress necessarily barred other changes in family-planning regulations proposed by the administration.

Those changes, also struck down yesterday, would have required clinics to obey any state laws requiring parental notification or consent and would have based financial eligibility on parents' resources rather than on the teen-ager's resources.

Wright's opinion, joined by Judge Harry T. Edwards, said it was "elementary" under the Constitution that state laws cannot conflict with federal laws. The income eligibility changes were simply another way of securing parental consent, it added.

The notification rule was issued after a year of debate and one of the largest public responses in the history of HHS. More than 120,000 individuals and organizations commented on the regulation after it was proposed last year.

Representatives of family-planning groups yesterday said they were pleased with the decision, but supporters of the rule vowed to fight for parental notification if the administration does not appeal or if it eventually loses all appeals.

Gordon Jones, executive director of United Families of America, which favored parental notification, said yesterday that his organization would consider suing the government to force the courts to address the issues of parental rights and religious and constitutional objections to government dispensing of prescription birth control devices without parental notice or consent. Those issues were not addressed in yesterday's ruling.

Sen. Jeremiah A. Denton (R-Ala.), a leading supporter of the notification rule, said he believed the decision "should be reversed by the Supreme Court or, that Congress should change the law to make it impossible for the courts to stand in the way" of administration efforts to ensure parental notification.

The decision yesterday was the latest in a recent series of major defeats for the administration in the federal courts. Those setbacks include recent Supreme Court decisions against it in the Bob Jones University racial discrimination case and a reversal of its decision not to require air bags in automobiles.

Other federal courts have overturned Reagan administration trade regulations designed to stop Americans from traveling to Cuba, actions involving the detention of Haitian immigrants and regulations designed to protect babies born with severe defects from being denied food or medical treatment.