The national meteorological service said France recorded its hottest nights on Tuesday as the heat indicator reached 21.6 degrees Celsius.
The institute said the national nighttime heat indicator was calculated from data at 30 meteorological stations across the country, noting that Monday and Tuesday were the hottest in France since 1947.
According to the meteorological institute, 54 departments in mainland France were placed under red heatwave alert, the highest alert level, on Tuesday, while 35 departments were placed under orange alert.
French Prime Minister Sebastien Lecornu said around 40 people have drowned in France since June 18, mostly young people, news channel BFMTV reported.
He also said that an interministerial crisis unit has been mobilised to respond to the heatwave.