Champagne production is expected to drop by 17% this year, it has been announced.
The French agriculture ministry said the dramatic fall was a result of unusually hot weather in the Champagne region in the northwest of the country.
The slump in production has sparked fears of higher prices for UK Champagne lovers.
The decrease was part of a nationwide fall in French wine production, the agriculture ministry said, as vines were damaged by spring frosts, drought and hail.
It said overall French wine production will fall by 12% this year.
However, the French government insisted the quality of wine will remain generally good.
In its second estimate for 2019, the ministry forecast production of 43.4 million hectolitres, down from 49.4 million in 2018 when output was boosted by good weather conditions.
Last month, the ministry put French output in 2019 in a range of 42.8 million to 46.4 million hectolitres.
A hectolitre - or 100 litres - is the equivalent of about 133 standard wine bottles.
In many vineyards, flowering occurred in rainy and cold weather, while heat and hail have also contributed to a decline in production potential, the ministry said.
Record-breaking heatwaves in June and July affected some regions in the south, such as the Gard, Herault and Var, causing burns on the grapes and production losses, it said.
Françoise Peretti, director of Champagne Bureau UK, said the region has a system called “Champagne Reserve” which allows it to “face adverse matters such as weather issues”.
She said: “Currently, the reserve, which is capped to 8,000 kilos/hectare, registers a comfortable 7,750 kilos/hectare.
“Producers have the leeway to collectively or individually release wine from their reserve should they need to.
“Current Champagne stocks also register a comfortable 1,392 billion bottles level.
“The region and its producers look forward to the 2020 harvest which is likely to start as early as next week and will continue well into September.”
By contrast, in some vineyards, early rainfall in August have helped to limit the impact of heat, while hail storms caused damage in the Beaujolais area where, along with Burgundy, output is expected to fall 26% from last year. In Bordeaux, production will be down by 4%.
By mid-August, French vineyards had mostly caught up their delayed growth compared to 2018, when the harvest was one of the earliest, the ministry said.
French wine grapes are harvested in late summer and early autumn, and the ministry said its estimates were tentative given uncertainty over conditions until the harvest.