Paris - Paris and other areas of northern and north-central France were on red alert for extreme heat on Thursday.
The capital's all-time record high temperature of 40.4 degrees, dating back to 1947, is likely to be beaten, according to state forecasting service Meteo France.
The forecaster predicts temperatures of 42 to 43 degrees in the Paris, Loire Valley and Champagne regions.
Authorities are warning residents to avoid all sports and any unnecessary trips, drink plenty of water, avoid alcohol, and check on elderly relatives and neighbours.
Wednesday night was likely the hottest ever in France, Meteo France said, with the average overnight low temperature provisionally calculated at 21.4 degrees.
The cities of Lille and, further south, Toulouse and Bordeaux recorded their highest-ever night-time minimum temperatures, the forecast said.
It is the second time ever for France to issue a red alert - the highest level of a three-step warning system - for a heatwave.
The first time was in June this year, when a record-breaking 46 degrees was measured in Vergargues, near Montpellier in southern France.
The previous record was 44.1 degrees, dating back to a 2003 heatwave that caused 15,000 more deaths than usual in France and an estimated 70,000 more across Europe.dpa