Chile's Constitutional Court has legalised abortion in exceptional cases, ending the country's absolute ban on the procedure.

A 6-4 ruling by judges on Monday accepted the constitutionality of making abortion legal in cases of rape, threat to the mother's life or deadly birth defects.

The Senate had approved the abortion bill last month, with President Michelle Bachelet vowing to sign it into law. The legislation was passed in its final form earlier in August, but conservative legislators then challenged its constitutionality.

Monday's judgement dismissed challenges brought forward by conservative political parties opposed to the reform, Rodrigo Pica, the court's secretary-general, told a news conference.

With the court's ruling on Monday, the bill has now become law.

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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.

Before Monday, the South American country was part of a small group of socially conservative nations that barred abortion under all circumstances - including the Dominican Republic, El Salvador, Gabon, Haiti, Malta, Nicaragua, the Philippines and Senegal.

Under the previous law, abortion in all circumstances was punishable by up to five years in prison.

Polls in July showed that 70 percent of Chileans support legalised abortion under the three conditions mentioned in the bill.

About 30,000 provoked or spontaneous abortions are recorded each year in the country, though it is estimated that clandestine abortions could number around 160,000, according to a pro-abortion rights group.

The Senate vote last month, approving the bill, was welcomed by Human Rights Watch (HRW), a US-based rights group.

"Chile's absolute prohibition on abortion has been a cruel law and bad public policy," said Jose Miguel Vivanco, HRW's Americas director.

"Doctors will no longer have to turn away women who are in desperate and life-threatening situations to seek clandestine or unsafe procedures," he said.