Located on the Channel coast near Dunkirk, Gravelines is the largest nuclear power plant in western Europe, with six 900 megawatt reactors.

The reactors were shut down on Sunday and Monday after a swarm of jellyfish clogged pumps used to cool them. The rare incident took place as a heatwave baked much of Europe.

"Reactor No. 6 restarted at 7:30 am this morning," an EDF spokeswoman told AFP. Work was still going on to restore three other reactors "in the coming days", the spokeswoman added. The plant's two other units are offline for maintenance.

The incident had not affected the safety of the facilities, personnel or the environment, the operator said.

The Gravelines plant was also disrupted by jellyfish in 1990. There have been cases of plants in other countries shutting down due to jellyfish, including in the United States, Sweden and Japan.

Experts say overfishing, plastic pollution and climate change have created conditions allowing jellyfish to thrive and reproduce.