Paris, Bretagne, Pays de la Loire, south of Auvergne, all along the German and Swiss border. And big cities in general (with the notable exception of big cities in the south and the Lille urbanisation).

Those places got more than 50% of "yes" for the referendum on the UE constitution in 2005.

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Source is https://www.interieur.gouv.fr/Elections/Les-resultats/Referendums/elecresult__referendum_2005/(path)/referendum_2005/index.html/referendum_2005/index.html) and also page 249 of "L'Archipel Français" by Jérôme Fourquet that was lying next to me.