The delay will also add 500 million euros to a project whose total cost is now estimated at around 13 billion euros ($13.8 billion), blowing past the initial projection of 3.3 billion euros when construction began in 2007.

It comes as EDF is already struggling to restart dozens of nuclear reactors taken down for maintenance or safety work that has proved more challenging than originally thought.

EDF also said Friday that one of the two conventional reactors at Flamanville would not be brought back online until February 19 instead of next week as planned, while one at Penly in northwest Farnce would be restarted on March 20 instead of in January.

The French government has warned of potential power shortages this winter because of the shutdowns at around two-dozen of the 56 reactors across the country that normally generate around 70 percent of its electricity needs.

EDF said the latest problems at Flamanville, on the English Channel in Normandy, emerged last summer when engineers discovered that welds in cooling pipes for the new pressurised water reactor, called EPR, were not tolerating extreme heat as expected.

As a result, the new reactor will be start generating power only in mid-2024.

The French-developed European Pressurised Reactor was designed to relaunch nuclear power in Europe after the 1986 Chernobyl catastrophe in Russia, and is touted as offering more efficient power output and better safety.

But similar projects at Olkiluoto in Finland, Hinkley Point in Britain and the Taishan plant in China have also suffered production setbacks and delays, raising doubts about the viability of the new technology.

French President Emmanuel Macron said in February that he wants a nuclear "renaissance" that would see up to 14 new reactors in France as the country seeks to reduce use of fossil fuels.