SCEC

The central civil registry service, based in Nantes, keeps and issues the civil status records of French people born abroad. After a naturalization it is the body that establishes your French birth certificate.

According to service-public.gouv.fr, the request is made online or by post to the central civil registry service in Nantes; the service receives no visitors. The page states a target of twenty days for an online request, and about thirty days by post. A certificate can only be issued for a person who is a French national.

That certificate is the document you will then be asked for when applying for a first identity card or passport: a certificate issued on secure paper by the central civil registry service can establish French nationality. That is why this step usually comes before the others once the decree has been published.