A gas plant in southern France was at risk of going offline late on Thursday as high temperatures in the Mediterranean Sea limited access to cooling water, piling further pressure on an energy system already facing reduced nuclear output.

Successive early summer heatwaves in Europe have seen temperatures rise to unprecedented levels, causing water shortages, wildfires, and some deaths.

French utility EDF issued a production restriction notice for the 930-megawatt Martigues plant, marking the first time this summer that a French gas-fired power plant was at risk of going offline due to the heat.

EDF told media that an exemption was granted to allow the gas plant to continue operations past the normal 30°C (86°F) water temperature threshold until September 15, but the extended limit of 32°C was already at risk of being breached.

The outage would add to the 4.9 gigawatts of unavailable nuclear capacity due to high river temperatures on Thursday evening. An additional 2.5 GW is offline due to low river levels.

Temperatures expected to fall, but drought worsening. Gas-fired power makes up only a small portion of the overall French energy mix, while nuclear energy makes up about 70% of the country's total supply. The nuclear outages represent about 14% of France's total capacity.

"We have seen two waves of climate-related unavailabilities that were unprecedented in both severity and timing, involving reactors that are not normally affected," Thibault Laconde, founder of climate data analytics company Callendar, told media.

Additional nuclear reactors are expected to regulate their output throughout the day on Friday due to the high temperatures, while an outage of the Bugey 3 reactor is expected to end on Friday evening as temperatures cool.

A heat-related complete outage at the Golfech 2 nuclear reactor in southern France is expected to end on July 25.

Around 2.5 GW is offline at the Chooz plant due to low water levels on the Meuse river, which is subject to a water-sharing agreement with Belgium.

France's heatwave is expected to continue to recede in the coming days, with high temperatures expected to be mostly confined to the southeast by the weekend, MeteoFrance said.

Due to the lack of significant rainfall, combined with exceptionally high temperatures, however, the drought has worsened day after day since the end of May, it added.