Washington ? In a surprise move with election-year implications, the Obama administration’s top health official overruled her own drug regulators and stopped the Plan B morning-after pill from moving onto drugstore shelves next to the condoms.

The decision by Health and Human Services Secretary Kathleen Sebelius means the Plan B One-Step emergency contraceptive will remain behind pharmacy counters, as it is sold today — available without a prescription only to those 17 and older who can prove their age.

The Food and Drug Administration was preparing to lift the age limit on Wednesday and allow younger teens, who today must get a prescription, to buy it without restriction. That would have made Plan B the nation’s first over-the-counter emergency contraceptive, a pill that can prevent pregnancy if taken soon enough after unprotected sex.

But Sebelius intervened at the eleventh hour and overruled FDA, deciding that young girls shouldn’t be able to buy the pill on their own — especially since some girls as young as 11 are physically capable of bearing children.

“It is common knowledge that there are significant cognitive and behavioral differences between older adolescent girls and the youngest girls of reproductive age,” Sebelius said. “I do not believe enough data were presented to support the application to make Plan B One-Step available over the counter for all girls of reproductive age.”

The move will anger a pivotal part of Obama’s Democratic base, and Sen. Patty Murray of Washington, a member of the Senate leadership, quickly asked for Sebelius to explain her decision. The White House said Sebelius decided on her own.

But the move also could help Democrats make their case to independents, whose support will be critical in next fall’s presidential election, that Obama is not the liberal ideologue Republicans claim. It followed Obama administration reversals this year on some environmental and other issues that irked Democrats.

It was the latest twist in a nearly decade-long push for easier access to emergency contraception, and the development shocked women’s groups and maker Teva Pharmaceuticals, which had been gearing up for over-the-counter sales to begin by month’s end.

“We are outraged that this administration has let politics trump science,” said Kirsten Moore of the Reproductive Health Technologies Project, an advocacy group. “There is no rationale for this move.”

“This decision is stunning. I had come to believe that the FDA would be allowed to make decisions based on science and the public’s health,” said Susan Wood of George Washington University, who served as the FDA’s top women’s health official until resigning in 2005 to protest delays in deciding Plan B’s fate. She said, “Sadly, once again, FDA has been over-ruled and not allowed to do its job.”

But the decision pleased conservative critics of the proposal.

“Take the politics out of it and it’s a decision that reflects the concerns that many parents in America have,” said Wendy Wright, an evangelical Christian activist who has helped lead the opposition to Plan B.

“This is the right decision based on a lack of scientific evidence that it’s safe to allow minors access to this drug, much less over the counter,” said Sen. Charles Grassley, R-Iowa.