People are being urged to avoid the Alps as a heatwave is expected to hit this week, making climbing conditions dangerous.
Local authorities in the Haute-Savoie region, which includes the French side of Mont Blanc, warned there are higher than usual risks of rockfalls and also new crevices opening up on glaciers.
It comes as a so-called "heat dome" is due to hit continental Europe. Italy, Switzerland and France have all issued heat warnings as temperatures are anticipated to hit 40C this week.
Haute-Savoie authorities said in a statement online: "We appeal to everyone's sense of responsibility and judgement and urged you, where possible, to delay taking the Normal Route up Mont Blanc."
In normal conditions, between 100 to 200 climbers reach the summit each day. Swiss weather authorities said on Monday that a new record had been set for the altitude of the zero-degree line in the Alps, which is the height at which the temperature dips below 0C.
According to Le Monde, it was clocked overnight from Sunday to Monday at 5,298 metres, "which constitutes a record since monitoring began in 1954".
Last July during a more severe heatwave, all seven routes to the Mont Blanc summit had deteriorated so much that only extremely experienced climbers could make. French authorities also closed down two popular mountain shelters due to the risk of rockfalls.
Four southern regions - the Rhone, Drome, Ardeche and Haute-Loire - were placed under "red alert" for heat on Monday. This allows local authorities to call off sports and cultural events and close public facilities if needed.
Meteo France said: "The heatwave is expected to peak between Tuesday and Thursday, depending on the regions. The length and intensity of this event could require stepping up alert levels in some departments.”
Meanwhile, grape-pickers in wine-producing regions of southern France have been advised to start work on the harvest in the early hours of the morning to avoid sweltering in a late summer heatwave.
Jerome Volle, a wine producer in Ardeche and vice-president of the French farmers' union FNSEA, told BFM TV that the grape harvest was already underway in his region.
One way to cope with the heat was to "start picking the grapes at 3.30 a.m. in the morning and to stop at 11am," he said.
Agriculture Minister Marc Fesneau also told reporters during a visit to a vegetable farm in Burgundy that the heatwave created problems for crops and so there was a need to adapt.
"To grow enough food, it is necessary to adapt the schedules, which farmers do, which market gardeners do, which breeders do, and at the same time continue to produce in conditions which are necessarily somewhat degraded," he said.
