A huge fire broke out at the 19th-century basilica of Saint-Donatien-et-Saint-Rogatien in Nantes, Brittany, western France, on Monday morning.
Images of smoke rising high into the Breton sky have been circulating on French social media.
The blaze was largely concentrated on the roof of the church.
The area was cordoned off, and around 40 firefighters were dispatched to the scene, the local fire and rescue department told FranceTV Info.
Frederic Leguiller, head of Nantes' fire operations, told Ouest France that the blaze began accidentally:
We were alerted mid-morning to a small fire in the church. Workers were working on the roof at that time. They tried to extinguish the fire immediately, without success unfortunately. Significant resources have been deployed to try to contain the fire on the roof of the church, which is not easy because of the very large volume.
He added that the "origin of the fire is still unknown."