Hailstorms and other severe weather have caused serious damage in some French wine-producing regions over the weekend.
Cognac was seriously hit, with up to 8 percent of the vineyard badly damaged, producers say.
Other regions hard hit where Chablis and Beaujolais.
"These hail bursts ravaged many vineyards, making future harvests uncertain or impossible," FNSEA, France's largest farm union, said in a statement on Monday.
Cognac producers say 5000 to 6000 hectares, amounting to 6-8 percent of the vineyards, were badly damaged by hailstones and heavy showers on Friday evening.
"There are places where they have been 100 percent destroyed," a spokeswoman for Cognac producers' group BNIC said.
Some 27.5mm of rain and nearly 15cm of hail fell in 15 minutes on affected areas, she said. Some wine producers reported hailstones the size of a quail egg, or about 1.5cm long.
In Burgundy, hailstorms and rainfall that followed on the night of Friday to Saturday damaged about 600 hectares, mainly in the southwest of Chablis -- by far the largest vineyard in Burgundy -- and in Auxerrois, a spokeswoman for Burgundy wine group BIVB said.
"For Chablis it is most difficult because it is the third blow this year after frosts in late April, a hailstorm on April 13 and now this one," she said.
Burgundy has around 29,000 hectares of vines.
In the Beaujolais region, known for its popular Beaujolais nouveau, about 1000 hectares or 6 percent of the vineyards have been hit by the storms but the damage to vines is not known.
"Where the storm passed it was very violent," a spokeswoman for wine group Inter Beaujolais said. "It chopped vines and ground leaves."
The storms will reduce the 2016 harvest but also have an impact on future harvests in all three regions.
FNSEA called on the government to provide support for producers affected by the damage.