France has announced emergency measures to stave off possible power shortages due to the unusually hot weather affecting much of Europe.
France's river levels have dropped
Nuclear power plants will be allowed to discharge the water used to cool their reactors at higher temperatures than usual.
French environmentalists have condemned the decision.
Temperatures in Paris have been above 35C (95F) for more than a week, and on Sunday set a record night-time high of 25.5C (77.9F).
The punishing heatwave, which is affecting swathes of Europe, has claimed 50 lives in Paris in the past four days, doctors say.
The decision to allow power stations to expel hotter water was taken after an emergency meeting between government officials and energy experts.
Two states in southern Germany, Bavaria and Baden-Wuerttemberg, have also temporarily raised permitted water temperatures to allow their nuclear plants to keep running.
Nuclear plants pour water back into rivers, but usually once it has been cooled to safe temperature.
The French Environment Minister, Roselyne Bachelot, said the measure would be applied on a case-by-case basis, and would take account of the environment.
Officials in Switzerland have cut output rather than allow hotter discharges of water into the country's rivers.
In France, about 80% of the country's electricity needs are met by 19 nuclear power stations.
The increased use of air-conditioning and refrigeration has put a further strain on the nuclear plants, and the environment minister has warned that power shortages could not be ruled out if problems in the electricity supply continue.
Italians have already been living with power shutdowns.
The Dutch electricity grid operator has warned it may not be able to meet demands for power at peak times from Monday onwards.
It is the first time in nine years that a "code red" alert has been issued for the Dutch national grid.
Doctors say French hospitals are unable to cope with the numbers of patients suffering from the heat.
Patrick Pelloux, head of France's emergency doctors' association, attacked French Government officials for classifying scores of heat-related deaths in the capital as "natural".
"They dare to talk about natural deaths. I absolutely do not agree with saying that," said Mr Pelloux.
French doctors say hospitals cannot cope with heat sufferers
Health officials said there were no accurate statistics blaming the heat solely for the deaths.
On Monday, five people died in a forest fire in north-eastern Spain. Hundreds of people had to evacuate their homes, but have since returned.
It brings to 24 the number of people killed by the extreme weather in Spain in the past two weeks, most of whom were elderly victims of heat stroke.
Fresh fires were burning on Monday in Portugal, already devastated by some of the worst forest blazes in Europe.
Forecasters say Spain, Germany and France have at least a few more days of intense heat, while Italy is expected to sizzle until September.
Italy experienced record heat on Monday. The Italian Meteorological Society said Turin reached 41.6C (107F) - the hottest day since records began 250 years ago.