Niger opposition leader and former Prime Minister Hama Amadou lost an appeal against a one-year jail term for baby smuggling, legal sources said.

Amadou - who lives in France - was among a group of around 20 people, including one of his wives, who received one-year terms in March 2017 for allegedly smugging babies from Nigeria via Benin to wealthy couples in Niger.

He was tried and convicted in absentia, and would have to return to Niger in order to serve his sentence.

The case has become a long-running political saga in the country, with Amadou's lawyers claiming the prosecution is aimed at sidelining President Mahamadou Issoufou's biggest rival.

Amadou, nicknamed "the Phoenix" for his political comebacks, challenged Issoufou in Niger's presidential elections in 2016.

He was arrested on November 14, 2015, on his return from exile, and was forced to campaign for the presidency from behind bars.

He was released on medical grounds on March 16, 2016, four days before the second round of the voting, and flew to France.

Issoufou went on to win with 92 percent of the runoff ballot. With the opposition boycotting the election, Amadou only got seven percent of the vote.

Constitutional lawyer Amadou Boubacar confirmed that under Nigerien law, Amadou was now barred from elective office.

"His appeal has been rejected, which means that his conviction is definitive," Boubacar told AFP. "Unless and until he is declared innocent, he will not be entitled to vote or run for office."