The parliamentary report, commissioned by a body looking into renewable energy, said that dismantling the sites could take longer than EDF had envisaged.

"The optimistic scenarios upon which EDF has based its provisions, as well as a certain number of major costs which have been omitted, lead to questions over the validity of EDF’s estimates, while at the same time, certain other costs appear to have been underestimated," the report said.

State-controlled EDF rejected those findings, and pointed out that the French Environment, Energy and Sea Ministry had backed EDF's estimates earlier this month.

"The Ministry concluded that the audit essentially backs up EDF's estimate of the cost of dismantling its nuclear fleet," EDF said in a statement.

EDF is the world's biggest nuclear power operator. It currently manages 58 French reactors which provide over 75 percent of France's electricity.

(Reporting by Benjamin Mallet and Sudip Kar-Gupta; editing by Jason Neely)