Up to 4.6m bottles of Spanish rosé wine have been labelled as French and sold in French cafes, hotels and restaurants, officials say.
The findings come from France's anti-fraud directorate after a two-year inquiry into producers, importers, traders and distributors.
While most wines were correctly labelled, up to 3.45m litres of Spanish wine was disguised as French, the reports says (in French).
Labels were either deceptive or false.
The most common offences involved obscuring the true origin of the wine or sticking a French symbol such as a tricolour on the bottle with labels such as "Produced in France" or "Bottled in France".
Other transgressions involved hiding the true origin on bag-in-box wine on the underside of the packaging or even beneath the handle.
The revelations are inflammatory, especially for wine-producing areas of southern France that have battled cheap imported wine from Spain.
Responding to reports that the amount of wine involved was as much as 7m litres, the head of the young wine-producers organisation in the Hérault region, Remi Dumas, said: "For those who wonder why we demonstrate, for all the wine-producers who aren't bothered by our calls for protest! Consumers open your eyes! 10m bottles of fake rosé from Spain."
Pour ceux qui se demandent encore pourquoi nous manifestons, pour tous les viti qui ne se sentent pas concernés par nos appels à mobilisations! Consommateurs, ouvrez l’œil 😉 Dix millions de bout de rosé espagnol maquillées en vin 🇫🇷 https://t.co/7R2BeTleOy— Remi Dumas (@RemiDumas34) July 9, 2018
"Rosé-lovers beware," warned Le Parisien newspaper on Monday. "You're in danger of a nasty surprise at happy hour."
Le Parisien quoted one senior official as saying that as much as 7m litres of wine had been "Frenchified".
"We've identified fraud at four trader-producers," Alexandre Chevallier of the General Directorate for Competition Policy, Consumer Affairs and Fraud Control (DGCCRF) told the newspaper.
The government body behind the report says anyone found guilty of such fraud could face two years in jail.
Earlier this year the French anti-fraud body revealed that as many as 66.5m bottles of wine had been passed off as Côtes du Rhône from 2013 to 2016, in a scam that affected UK drinkers as well as French.