According to a criminal investigation under way in the Bordeaux area, almost 200,000 bottles of the chateau's 1995 second string vintage - worth pounds 30 or more in the shops - were doctored with milk, water and fruit acids and adulterated with a cheaper, local red wine.
Several employees of the chateau - one of the best respected names in the Margaux area of the Bordeaux vineyards - have been placed under formal examination for fraud.
According to the newspaper Le Monde, the Dutch millionaire who runs the chateau, Eric Albada-Jelgersma, has also been questioned and may also face criminal charges. In an interview with the newspaper yesterday, a former director of wine-making at the chateau admitted that he had improved and increased the domaine's 1995 Margaux by mixing it with cheaper wine from a vineyard over the road. Jean-Michel Ferrandez said he knew this was illegal but he said "lots of people do it".
Such blends utterly defeat the principle of Appellation Controlee - restrictions based on the recognised qualities of vineyards - upon which all French wine-making is based. The judicial investigation is also examining evidence, drawn from the chateau's own carefully-kept records, that the domaine's 1995 Margaux was doctored in other ways: with milk (to improve the scent or bouquet), water (to increase quantity) and acids (to improve the taste).
The allegations will send shock waves through the French wine industry. There have been similar scandals from time to time, but none since the 1970s involving one of the top names in Bordeaux.
The premier product from the Chateau-Giscours is a Grand Cru Classe - in other words it is classified as one of the very best wines of Bordeaux.
The product at the centre of the allegations is the chateau's second grade wine - a still highly-respected, and highly-priced product, which is labelled as Margaux, Mise en Bouteille a Chateau-Giscours.
The allegation will add to criticism of the organisation which is supposed to police the quality of French wines, the Institut National des Appellations Controlees (INAO).
The INAO has been accused of sometimes allowing inferior wines to be sold under its control, undermining the whole concept of graded wine-growing localities (terroirs) upon which France lays claim to make the world's finest wines.