The court is to decide on a law that legalizes abortion in limited circumstances and end the socially conservative country's status as the last in South America with a blanket ban on the procedure.
Before the adoption of the new law and the court's ruling, Chile was one of very few countries in the world where abortion is criminalized with no exceptions. "We were strong in the legislative debate, we were convincing in front of the constitutional court and the constitutionality of the three grounds has been recognised".
Anti-abortion protesters voice their dissent in Santiago.
The Chilean senate approved a bill in July to allow abortion under those circumstances, but conservative lawmakers challenged its constitutionality.
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The Chilean reproductive rights organization Miles hailed the court's decision as a "historic moment that marks a before and after for Chilean women".
The ruling ended what was a series of over two years of lawsuits and controversy, giving women's rights groups a sound victory.
The court heard the arguments of more than 130 concerned organisations before making its ruling, which removes the last major hurdle for the bill to become law.
Chile legalised abortion for medical reasons in 1931, but the procedure was then banned under all circumstances in 1989 during the dictatorship of General Augusto Pinochet. Legislators introduced over a dozen bills to partly legalize abortion as early as 1991, but all were shelved or rejected.
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Supporters had hoped Bachelet could sign the new laws before the November presidential elections in which she is constitutionally barred from participating.
The move to ease abortion restrictions is seen as her swansong as she leaves office.
In a statement, the Roman Catholic conference of bishops in Chile said the ruling, which can not be appealed, "offends the conscience and common good of our citizens". In the four years up to 2014, 73 people were jailed.
Chile is one of four countries that now prohibit abortion in all cases, according to the New York-based Center for Reproductive Rights, though a few others have rules so restrictive that they amount to de facto bans. The news service reports up to 70,000 illegal abortions took place annually in Chile, often with the aid of pills obtained on the black market.
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