The lawyer says the man 'is consumed with remorse'.
A church volunteer admitted being behind a fire that severely damaged Nantes' St. Peter's and St. Paul's Cathedral on July 18 and was charged with arson.
The 39-year-old man was taken into custody and questioned shortly after the incident due to the fact that he had been closing the cathedral on Friday evening.
He got released on Sunday 19, but was taken into custody again on Saturday 25 and later charged with "destruction and damage by fire" before being put in pre-trial detention.
Lawyer Quentin Chabert told local newspaper Presse-Océan that his client is "consumed with remorse" and said he was "liberated" after admitting the incident.
The suspect told the examining magistrate he lit three fires in the cathedral, which resulted in the severe damage of a 17th-century organ and stained glass windows.
He could now be handed "a 10-year prison sentence and a fine of €150,000 euros," Nantes prosecutor Pierre Sennès explained.
The rector of Nantes cathedral, Father Hubert Champenois, said last week that the volunteer was a Rwandan citizen who came to France as a refugee "a few years ago".
He said he had been serving "as an altar" and had known him for "four or five years".
"Most of the works" have been saved from the fire and are now being stored in the castle of Nantes, said Philippe Charron, head of the heritage department at the Regional Directorate of Cultural Affairs (DRAC). But "very few" elements of the organ, "if any at all, will be saved", he added.
Authorities are now assessing the reconstruction process "stone by stone".
Prime Minister Jean Castex, who had come to Nantes to praise firefighters work on the day of the fire, said the state will fully take part to the works.
President Emmanuel Macron too had praised the effort of the emergency services to save "the jewel of the city of the Dukes".
The Gothic building's construction dates back to 1434 but works were completed only in 1891.
After a huge fire in 1972 that destroyed the roof, the cathedral had been renovated with concrete roof beams.
The renovation was only completed in 2013.