Niger's military rulers, who seized control of the government in July, gave the French ambassador 48 hours to leave the country, Niamey's foreign ministry said in a statement Friday.
Relations between the new regime in Niamey and several Western powers, as well as the West African bloc ECOWAS, have deteriorated since the coup on July 26.
France's government quickly rejected the order against its ambassador, repeating that it did not recognise the military rulers' authority.
The French foreign ministry told AFP on Friday evening: "The putschists do not have the authority to make this request, the ambassador's approval coming solely from the legitimate elected Nigerien authorities."
The ultimatum against the French envoy comes days after ECOWAS threatened military action to reverse last month's coup, which overthrew president Mohamed Bazoum.
Paris has repeatedly backed calls by ECOWAS for the reinstatement of Bazoum.
France has 1,500 soldiers based in Niger who had been helping Bazoum in the fight against jihadist forces that have been active in the country for years.
- Pressure from ECOWAS -
Earlier Friday, the Economic Community of West African States (ECOWAS) urged Niger's coup leaders to reconsider their position and pushed for a return to civilian rule, with the threat of force still "very much on the table".
While the generals who ousted Bazoum have called for a three-year transition period, ECOWAS demands the immediate return to constitutional order.
With delegations shuttling into Niamey, ECOWAS said negotiations remained its priority as defence chiefs prepared a standby mission for a possible "legitimate use of force" to restore democracy if needed.
"Even now, it is not too late for the military to reconsider its action and listen to the voice of reason as the regional leaders will not condone a coup d'etat," ECOWAS commission president Omar Alieu Touray told reporters in Abuja.
"The real issue is the determination of the community to halt the spiral of coup d'etats in the region."
ECOWAS has already applied sanctions against Niger to pressure the new regime.
Three other governments have fallen to military rebellions in the Sahel region since 2020, and jihadists control swathes of territory.
ECOWAS leaders are already negotiating with military administrations in Mali, Burkina Faso and Guinea who are all working towards transitions to democracy after their own coups.
After initially balking, Niger's new rulers have said they remain open to negotiations.
But they have sent mixed messages, including a threat to charge Bazoum -- who remains detained at his official residence with his family -- with treason.
- Aggression -
Niger's military leaders have also warned against any intervention, accusing ECOWAS of preparing an occupying force in league with an unnamed foreign country.
The officers on Thursday said they would let troops from neighbouring Mali and Burkina Faso intervene in Niger in case of an aggression.
But Touray dismissed plans for ECOWAS "declaring war" or an "invasion" of Niger, insisting the standby mission would be a legitimate force allowed under ECOWAS statutes agreed by members.
"The instruments include the use of force. So it is very much on the table, as are other measures we are working on," he said.
"If peaceful means fail, ECOWAS cannot just fold its hands."
ECOWAS has intervened militarily in past crises, including in civil wars. Few details of the new standby force have emerged.
But preparations for any possible use of military force in Niger are risky and already face political resistance in northern Nigeria, a key player in ECOWAS and the region.
Niger's northern neighbour Algeria has also warned of disastrous consequences for the region from an intervention.
Foreign Minister Ahmed Attaf this week toured West African countries to try to find a solution to a crisis in which Algiers firmly opposes any military option.
"There is a time for everything and we are currently in the time of finding peaceful solutions," he said on a visit to Benin.
"Let's put all our imagination into giving every chance to a political solution."