Météo France took the stop of increasing the alert level from orange to red with the worst of the heatwave still to hit the Paris region and northern France.
As well as Paris and its surrounding départements in Île-de-France, other parts of the north on red alert included parts of Normandy and the Hauts-de-France region.
Some 60 départements were still on orange alert, with only Brittany escaping the sweltering heatwave.
The red alerts will be in place until Thursday when temperatures are set to peak in northern France. The all-time high temperature recorded in Paris of 40.4C could well be broken.
Météo France believe temperatures in Paris could reach 42C on Thursday.
BREAKING – 20 départements en vigilance rouge à partir de 16h (Meteo France) pic.twitter.com/uXWjcsDgN6
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France's Health Minister Agnes Buzyn had a stern warning for members of the public who could suffer health problems as a result of the extreme heat: “Everyone is at risk with these kinds of temperatures.”
She warned that it was ill-advised to take part in any sporting activity over the coming days.
It's only the second time Météo France has issued red level warnings for a heatwave in France. The first time was during June's heatwave when several départements in the south were place on red alert as France recorded its highest ever temperature of 46C.