The fire in Nantes is "probably linked to waterproofing work that was taking place on the roof," said the city's chief fire officer, who added the blaze had been "contained" by mid-morning.
Some 60 firefighters fought the inferno at the basilica Saint-Donatien, which broke out around 10:45am (0845 GMT). Two workers on the roof when the blaze broke out escaped injury.
The back part of the basilica has been seriously damaged and some of the wooden beams have given way, authorities said.
Video footage of the blaze showed huge flames licking the rear part of the basilica, with the whole central section of the church gutted.