French authorities were on Thursday investigating the deaths of four people who were harvesting grapes in the famed Champagne region, as locals suspected they suffered sunstroke in unusually high outdoor temperatures.

Prosecutors in the cities of Reims and Chalons-en-Champagne said two men died in recent days while picking grapes, one woman died at home a few days after feeling faint during her vineyard work, while a fourth died in hospital after falling from a straddle tractor without showing any physical injuries from the fall.

None of the deaths was being considered suspicious and no autopsies ordered, prosecutors said.

They declined to comment on any possible link to high temperatures, reported at up to 34 Celsius (93F) in the region at the end of last week.

But winegrowers said the heat might be to blame.

"Maybe it will turn out that the sun had something to do with this," said Maxime Toubart, head of the Champagne growers' association.

"I am very sad," he told AFP. "People don't join the harvest to lose their lives."

Some 120,000 people were helping with the two-week harvest every year and "obviously you're going to have some accidents", he said. Every year, "one or two people" died from heart failure or aneurysms, he said.

One risk factor was the lack of physical preparedness for what was a demanding job, he said.

"More and people come here without being in the physical shape needed for outdoor work. Some young people don't have breakfast, don't hydrate, are on medication or working shirtless," he said.

Grapes for champagne are grown on 34 hectares in eastern France, where over 16,000 growers produce over 300 million bottles of champagne each year.

The United States are the main export market for champagne, followed by the United Kingdom, Japan and Germany.