After 28 months of growing controversy, the Food and Drug Administration yesterday indefinitely postponed its decision on whether women should be allowed to buy the "morning-after pill" Plan B without a prescription -- despite earlier assurances that it would act by Thursday.

The decision to embark instead on a new regulation-writing process was immediately denounced by women's health advocates and some lawmakers as a stalling tactic to achieve political and ideological ends.

FDA Commissioner Lester M. Crawford acknowledged that the agency's drug review staff had concluded the drug could be safely used as an over-the-counter drug by women older than 17. But in an unexpected twist, Crawford also said the application raised complicated and unresolved issues about whether current regulations allow a drug to be legally sold by prescription only for teenagers but over the counter for all others.

"What we're saying today is that there are unique regulatory issues here that need to be addressed before we can take a final action on the application," Crawford said. He said he could not estimate how long that might take.

Sens. Hillary Rodham Clinton (D-N.Y.) and Patty Murray (D-Wash.) said they were incensed because they had allowed Crawford's nomination as commissioner to move forward this summer only after getting a promise that a decision on the Plan B issue would be made by Thursday.

"I am stunned and outraged and furious," Murray said. "This is not only a broken promise to us, but another frightening example of politics trumping science at the FDA."

Many social and religious conservatives oppose easier access to emergency contraception and have flooded the White House with their views in recent days. Some oppose the easier access because they believe it will encourage promiscuity among young girls, and others because they believe emergency contraception can be a form of abortion.

Wendy Wright, policy director for Concerned Women for America, who has led the opposition, welcomed Crawford's announcement.

"I believe the FDA is being sensitive to the fact that this drug is not like others," she said. "How can you make it available OTC [over the counter] for most women and not have it get into the bodies of girls for whom it has not been approved without prescription? This will allow more people into the process of making that assessment."

Plan B is considered a contraceptive by the FDA and mainstream medical organizations, who define pregnancy as the point when a fertilized egg is implanted in the uterus. The FDA has also concluded that easier access to emergency contraception -- which generally must be taken within 72 hours of a contraceptive failure or unprotected sex -- would substantially reduce the number of unwanted pregnancies and abortions.

But others who believe that pregnancy begins at the point that the egg is fertilized argue that drugs such as Plan B can cause an abortion, since in some circumstances they prevent a fertilized egg from implanting.

Plan B, which is a concentrated form of a standard contraceptive, was approved by the FDA in 1999 as a prescription-only drug. Four years later, an FDA advisory panel voted 23 to 4 in favor of an application from drugmaker Barr Laboratories Inc. to make Plan B available without a prescription.

The FDA rejected the application in May 2004, however, saying there was insufficient information on whether the drug is safe for use by girls younger than 16.

At the time, the FDA advised Barr Laboratories to submit another application that allowed over-the-counter sales only to women older than 17. That was the plan that was deemed to be impossible to resolve yesterday.

In his announcement, Crawford said a formal rulemaking process would begin immediately on the question of whether a drug can be prescription-only and over the counter for different age groups. He said the issue had significant regulatory implications, though he acknowledged that several drugs with such dual status are already on the market. He said the question of how to enforce the dual status in pharmacies is a central concern.

Advocates of emergency contraception charged that the decision to propose a new regulation was a rejection in disguise, done for political reasons.

They pointed to the fact that Rep. Tom DeLay (R-Tex.) and 45 other members of Congress wrote President Bush to lobby against easier access to Plan B, and that even some pro-choice Republicans have backed away from their previous support for the proposal. Govs. George E. Pataki (R-N.Y.) and Mitt Romney (R-Mass.), two potential candidates for president in 2008, recently vetoed bills that would have made Plan B available without a prescription in their states, citing concerns about how it would be used and whether it could cause abortions.

Karen Pearl, interim president of the Planned Parenthood Federation of America, called it a "a terrible day for women's health and safety. It is crystal clear that the FDA has chosen politics over legitimate science. . . . The FDA has failed its public health responsibilities and has failed women."

Ironically, for opponents of emergency contraception, the very public controversy over Plan B has apparently increased public awareness of it. Barr Laboratories said sales have about doubled since the company applied for over-the-counter status in April 2003.

Barr Chairman Bruce L. Downey said in a statement that the delay was "not justified" and that "we are disappointed that the FDA did not approve our application."

"In our submission to the FDA, we provided a detailed legal analysis supporting approval of a dual label product, and continue to believe that a dual label status can and should be approved for Plan B," he said.

In a joint statement, Clinton and Murray said: "For six months we have asked for a simple answer -- yes or no. It is a breach of faith to have this Administration give us their word that a decision would be made and have that promise violated."

They said Sen. Mike Enzi (R-Wyo.), chairman of the Senate Health, Education, Labor and Pensions Committee, had promised to hold a Plan B hearing if there were further delays in this application.

"We are calling on him to honor this promise and promptly schedule a hearing to look into FDA's approval process for this drug and to ensure that American patients are receiving access to safe and effective treatments in a timely manner," they said.

FDA staff had concluded Plan B could be safely used as an over-the-counter drug by women older than 17. FDA Commissioner Lester M. Crawford cited regulatory issues in announcing the delay in approval. That delay was quickly denounced.