Tumbling by more than 15 percent in mid-afternoon trading in Paris, EDF's stock massively undershot the CAC 40 index of leading shares, down 1.0 percent.

It was "quite a violent reaction, but to be expected" given the wave of bad news, said Alphavalue analyst Nicolas Bouthors.

The night before, EDF had warned that it would miss its targeted ratio of net debt to underlying profits, after the government ordered it to sell an additional 20 terawatt-hours of electricity from its nuclear plants to competitors at a lower cost.

Ministers hope to limit increases in consumers' power bills to four percent, as Europe faces an energy squeeze and France heads into an April presidential election where rising costs of living are expected to be centre stage.

Tapping EDF to cover the costs was "the only option available to the government in its political effort ahead of the election," Bouthors said.

Finance Minister Bruno Le Maire told newspaper Le Parisien that the measure would cost the power company -- more than 80 percent of which is owned by the government -- up to 8.4 billion euros ($9.6 billion).

With most of EDF's generation capacity for 2022 sold in advance, the firm will have to buy up electricity elsewhere and sell it on at a loss.

Major union CGT accused the government of "a new stage in looting EDF for the benefit of its competitors", saying Paris was "killing" the French landmark.

In a bid to soften the blow, Le Maire recalled that the state was also supporting EDF with plans for a new-generation fleet of nuclear reactors.

"We're launching a new programme with considerable prospects for EDF's development," Le Maire said.

- No quick fix -

EDF currently generates around 70 percent of France's electricity with its 56 nuclear reactors, giving the country a smaller carbon footprint than many comparable European neighbours.

But large parts of the fleet are nearing the ends of their 40-year working lives, with 10 reactors currently out of commission as problems are discovered or feared in the aging plants.

Also Thursday, France's IRSN nuclear regulator warned of possible safety problems with the cooling system at a third plant, after corroded welds on the pipes of an emergency cooling system were found or suspected at two others of similar design.

News of the discovery at the Penly plant was "especially troubling, because it has a different design" from those already shut down, analyst Bouthors said.

"The corrosion problems don't seem to be associated with just one type of reactor."

EDF is currently going over its entire nuclear fleet to identify any further potential problems.

"Checks will have to be made, maybe as reactors are taken offline according to schedule, but some might have to be brought forward," the deputy head of the ASN nuclear safety authority, Julien Collet, has said.

Looking to the future, EDF's big hope was a next-generation reactor design known as EPR, which would be the model commissioned if President Emmanuel Macron's nuclear plans go ahead.

So far the few EPRs built, including in Britain, Finland and northern France, have been plagued by enormous delays and cost overruns.

The group announced still further delay into 2023 and another 300 million euros in cost overruns for its project in Flamanville in northern France on Wednesday.

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