EDF had notified the watchdog last week about the incident, saying investigations had revealed a thinning of the metal in certain sections of pipes at the reactors, making it impossible to guarantee their resistance in the event of an earthquake.

The utility said that it could lead to flooding of the pump houses and a loss of the cooling systems of the reactors.

EDF had said 29 reactors were involved, and 20 were classified as "Level 2" on the international nuclear and radiological event scale (INES), where Level 1 marks the lowest level of risk while Level 7 is the highest.

ASN, which did not comment after EDF's notification last week, issued its statement on Monday, initially saying that the Level 2 alert concerned 29 reactors instead of 20, before correcting it.

ASN's statement sent French day-ahead and forward power prices higher.

There are growing concerns that France, which depends on atomic power for over 75 percent of its electricity needs, could experience tight supplies in the coming weeks due to prolonged outages at several reactors and reviews ordered by the regulator.

The regulator said repair works had started at all the reactors involved and it would monitor the work to make sure all repairs were completed.

It said that for those reactors currently in operation, repair works should be initiated immediately.

ASN did not say how long the repairs would last, nor whether it would lead to a halt in production. However, EDF reiterated that the repairs would not stop output and they would be completed on reactors that are currently on scheduled maintenance before their restart.