By Nellie Peyton

DAKAR, March 20 (Thomson Reuters Foundation) — Niger's top court has outlawed the practice of keeping women as maids and sex slaves known as "fifth wives", capping a decade-long legal battle by one victim that could inspire others in the West African nation to seek justice, lawyers and activists said.

The "fifth wife" custom — also known as "wahaya" — is when in addition to the four wives permitted by Islam, rich men take on other, unofficial wives who live as domestic and sexual slaves, said Britain-based group Anti-Slavery International.

Hadizatou Mani, a woman who was forced into slavery as a "fifth wife", escaped over a decade ago and married someone else, but her former master took her to court accusing her of bigamy, said Anti-Slavery International, which backed her case.

A Nigerien court initially ruled in favour of her master, but Mani appealed the decision. Last month, the Niger Court of Appeals ruled that her first marriage was never valid and that all "fifth wife" marriages are illegal, according to court documents seen by the Thomson Reuters Foundation.

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"The custom of "wahaya" or "sadaka" is a traditional practice of acquiring a young women to serve as both a maid and a "concubine" or sex slave," said the ruling, signed by the chief clerk of the Court of Appeals.

"This custom ... is contrary to the laws of the republic and the international conventions regularly ratified by Niger."

A spokesman at the justice ministry said it was pleased with the ruling, which was only circulated among activists last week.

Mani's lawyer, Abdourahaman Chaibou, said it will set a precedent for future cases.

"I think this decision is fundamental, in that it rejects a custom that has been practiced for centuries," Chaibou said.

"It's a victory for the defence of human rights and especially the fight against slavery."

No one knows how many women live as "fifth wife" slaves in Niger, but the practice is fairly common in some areas, said Jakub Sobik of Anti-Slavery International. The West African nation also has a tradition of descent-based slavery, he said.

About 133,000 people in Niger, a country of 20 million, live as modern-day slaves, according to the 2018 Global Slavery Index by the Walk Free Foundation.

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"This is a decision we have waited years for," Mohamed Mogaze, national coordinator for the Nigerien anti-slavery organisation Timidria, told the Thomson Reuters Foundation.

Activists will now focus on raising awareness of the ruling so that women can bring more cases to court, said Mogaze.

Mani was bought by her former master at the age of 12 for 240,000 CFA francs ($418) and was enslaved for nine years, according to the court ruling.

She first took her case to West Africa's regional ECOWAS court, which ruled in 2008 that Niger had failed to protect her from slavery, said Sobik of Anti-Slavery International.

This drove Niger to take the issue more seriously, he said.

"The fight is far from being won, but we're in it for the long haul," said Sobik.

"Together with our partners we will work to ensure a full implementation of this ruling."

($1 = 573.9300 CFA francs)

(Reporting by Nellie Peyton; Editing by Kieran Guilbert. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's and LGBT+ rights, human trafficking, property rights, and climate change. Visit http://news.trust.org)