The automatic shutdowns of four units "had no impact on the safety of the facilities, the safety of personnel, or the environment", EDF said on its website.

"These shutdowns are the result of the massive and unpredictable presence of jellyfish in the filter drums of the pumping stations," the Gravelines plant operator said.

The site was fully shut after the incident, with its two other units already offline for maintenance.

Teams were carrying out inspections to restart the production units "in complete safety", EDF said, adding the units were expected to restart on Thursday.

"There is no risk of a power shortage," the company added, saying other energy sources, including solar power, were operational.

Gravelines is Western Europe's largest nuclear power plant with six reactors, each with the capacity to produce 900 megawatts.

The site is due to open two next-generation reactors, each with a capacity of 1,600 megawatts, by 2040.

This is not the first time jellyfish have shut down a nuclear facility, though EDF said such incidents were "quite rare", adding the last impact on its operations was in the 1990s.

There have been cases of plants in other countries shutting down due to jellyfish invasions, notably a three-day closure in Sweden in 2013 and a 1999 incident in Japan that caused a major drop in output.

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