Article R2124-3 of the French General Code of Local Authorities
The formal notice sent by the prefect to the mayor or president of the public establishment for inter-municipal cooperation, in the cases provided for in article L. 2124-3, may be made either by letter, telegram or telephone message. The reply sent to the prefect by the mayor or president of the public establishment for inter-municipal cooperation must be made in one of the forms indicated in the previous paragraph.