GAITHERSBURG, Md.  A federal advisory panel voted unanimously Thursday that federal drug regulators should approve a medicine that could help prevent pregnancy if taken as late as five days after unprotected sex.

The pill, called ella, sprang from government labs and appears to be more effective than Plan B, a morning-after pill now available over the counter to women 18 and older that gradually loses efficacy after intercourse and can be taken at latest three days after sex. Ella, by contrast, works just as well on the fifth day as the first after sex.

Ella blocks the effects of progesterone, a female hormone that spurs ovulation. It is a chemical relative to RU-486, the abortion pill, and some mystery remains over exactly how it works. That mystery spurred a fierce debate outside the committee over whether it should be considered an abortion drug, a debate that prompted the posting of several uniformed police officers around the meeting room.

The F.D.A. usually follows the advice of its advisory panels but not always.

The dispute is whether the drug works by delaying ovulation (as the pill’s manufacturer claims) or by preventing a fertilized egg from implanting itself in the uterus (as anti-abortion advocates say).