With only two of the 10 judges voting against lifting the ban, the decision marks a small but historic shift in abortion law in Latin America's biggest country. Brazil, like many countries in the region, has long banned abortions in all cases except pregnancies caused by rape and those which pose a threat to the life of the mother.

While private hospitals and illegal clinics have long found ways around the ban, the decision now makes it possible for mothers carrying fetuses suffering from anencephaly to abort the pregnancy legally.

The measure applies specifically to cases of anencephaly, a disorder that leads to a malformation or absence of large parts of the brain and carries an overwhelming likelihood that the baby will die shortly after birth.

Such babies "would never become a person," said Justice Marco Aurelio Mello, speaking for the majority. "This is not about a potential life, but about certain death."

Abortion rights advocates and medical groups for years have pushed for such a measure, arguing that mothers who carried babies likely to die post-delivery should be spared unnecessary trauma.

"The diagnosis itself is bad enough," said Cristiao Rosas, a physician and spokesman for a Brazilian federation of obstetrician and gynecology groups. "It's a condemned gestation, with no prognosis for extra-uterine survival, and with a devastating impact on the psychological and emotional health of the mother."

Religious groups in Brazil, which still wield significant sway at the ballot box and in matters of public opinion, remain fiercely opposed to any changes to abortion law.

"We all have an absolute right to life from conception until natural death regardless of any type of deficiency," said Luiz Carlos Ludi, a Catholic priest who led a protest this week outside the Supreme Court building in Brasilia.

Such sentiments, echoed by Brazil's growing evangelical population, mean any bigger changes to abortion law in the country remain unlikely in the near future.

"This is a small and gradual step for very specific cases," said Rafael Cortes, a political analyst at Tendencias, a consultancy in Sao Paulo. "Change bigger than this would be much more difficult."

During Brazil's last presidential election, in late 2010, the debate over abortion helped derail what had appeared to be an easy first-round victory for President Dilma Rousseff.

After religious voters flocked to a born-again candidate in the days before the first vote, Rousseff had to backpedal from her abortion rights comments to assure her victory in a runoff.

(Writing and additional reporting by Paulo Prada; Editing by Jackie Frank)

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