Tens of millions of people in France sweated through a late summer heatwave on Monday, with record temperatures expected in the wine-growing Rhone valley region and a forest fire also blazing in the southeast.

Temperatures are expected to peak on Tuesday and Wednesday, with the most intense heat in the southern parts of the country where the mercury is already pushing past 40 degrees Celsius (104 Fahrenheit).

Health authorities have placed 50 out of 96 departments in mainland France in the second-highest heat warning level, with some areas expected to be placed in the maximum red category in the coming days.

"Some records could be broken, notably on Tuesday in the Rhone valley with 40-42C expected," national weather service Meteo France said.

The heatwave is "intense and long-lasting" and "particularly late in the season," it added.

After a suffocating summer of record temperatures and forest fires in 2022, France has for the most part experienced a typical holiday period this year, escaping the intense heatwave that scorched southern Europe in July.

The current high temperatures are the hottest of the season, Meteo France said.

Around 260 firefighters are currently battling a fire near the village of Chanousse in southeast France, according to the Association for the Prevention and Reporting of Forest Fires.

It has consumed around 100 hectares (nearly 250 acres) of forest.