Nearly a quarter of voters in Niger - 1.5 million people - will be barred from casting their ballots in next month's presidential poll, the country's government said, after politicians failed to agree on a system to register them.
Tensions were already running high in the unstable West African state which is a key Western ally in the fight against militants across the Sahel region.
Around 1.5 million of Niger's registered voters lack the documentation required to prove their identity, and the Independent National Electoral Commission had proposed a system whereby witnesses could vouch for them.
But politicians failed to approve it at a meeting on Friday.
"There was no consensus among the political class and therefore 1.5 million Nigeriens cannot exercise their right to vote on 21 February," Alkassoum Indatou, spokesman for the presidential majority, said on state TV on Friday night.
It was not immediately clear whether the registration problems would affect some parts of the country or political blocs more than others.
The opposition in the uranium producer accuse incumbent President Mahamadou Issoufou of being increasingly authoritarian - but the government has underscored the need for order amid security threats, including an alleged attempt to topple Issoufou last month.
One of the main opposition candidates Hama Amadou is in jail on charges of being linked to a baby-trafficking ring. He denies the charges and says they are politically motivated.
Another presidential candidate, Adal Rhoubeid, arrived back in the capital Niamey on Friday night after being detained for questioning in relation to attacks in a hotel and cafe in Brukina Faso's capital last week that killed 30 people, including many foreigners.
"Tragic events [in Ouagadougou] confirm something that is becoming more and more clear: terrorism is a cancer across the Sahel and Sahara," he said in front of a large crowd of supporters at the airport, adding that his MDR party supported compulsory military service.