A US federal judge has ordered the government to make the "morning after" pill available over the counter to girls of all ages within 30 days.
Judge Edward Korman said a decision by the US health secretary to limit over-the-counter purchases of the drug to those 17 and older was "capricious".
The reproductive rights group which brought the case called the ruling a victory for women.
A US government lawyer said it was considering legal options.
In Friday's decision, New York Judge Korman struck down the Food and Drug Administration (FDA) rule requiring girls 16 and under to have a prescription for the pill.
He said the FDA's restriction was "arbitrary, capricious and unreasonable".
Women all over the country
will no longer face arbitrary
delays and barriers just to
get emergency contraception"
Center for Reproductive Rights
The Center for Reproductive Rights, which brought the case, argued that there was no scientific proof that girls younger than 17 could not safely use the drug without supervision.
It said that politics had more to do with limiting access to the pill. US social conservatives have argued that widening the pill's availability would promote sexual activity among younger girls.
In 2011, the FDA said it had concluded that the "morning after" pill could be safely used by girls of child-bearing age.
But in an unprecedented move, Health and Human Services Secretary Kathleen Sebelius overruled the agency.
She said there was not enough evidence from all potential ages to support removing over-the-counter restrictions. President Barack Obama said he supported the decision.
In his ruling, Judge Korman criticised the Obama administration's justification, calling it "an excuse to deprive the overwhelming majority of women of their rights to obtain contraceptives without unjustified and burdensome restrictions".
Teva Pharmaceuticals, which manufactures the birth control drug, called Plan B, had also petitioned the government to lift the restrictions.
An FDA spokeswoman declined to comment on Friday's ruling, saying it was an ongoing legal matter.
F Franklin Amanat, a lawyer for the Department of Justice, told the Associated Press news agency: "We are reviewing the decision and evaluating the government's options."
The Center for Reproductive Rights welcomed the ruling.
"Women all over the country will no longer face arbitrary delays and barriers just to get emergency contraception," spokeswoman Nancy Northup told Reuters news agency.
Emergency contraception is effective for the first 72 hours after sex, but is more likely to work the sooner it is taken, and can cause side-effects such as nausea and diarrhoea in some women.