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Around 40 firefighters were deployed to the scene to try to extinguish the flames.
However the fire spread quickly, ravaging the rooftop of the historic building.
The blaze reportedly broke out at 10:30 local time (08:30 GMT), but the cause has not yet been established.
"I see flames still escaping," Rev Bertrand told BFM television.
Social media users shared images of the basilica, with smoke billowing from its rooftop.
rederic Leguiller, head of Nantes’ fire operations, told Ouest France that the blaze began accidentally
He said: “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.”