“The EPR is a prototype and prototypes are always hard”, Levy said, adding that the new cost estimate was realistic.

Britain’s first new nuclear power station for a generation has encountered yet another delay.

Professor David Elmes, of Warwick Business School, said: “News from EDF that the Hinkley Point C new nuclear power station won’t start in 2023 is hardly unexpected”.

EDF chairman and managing director Jean-Bernard Lévy said: “It is a priority for EDF and of critical importance for the French nuclear industry and its success internationally”.

“The new roadmap, to which EDF and its partners are committed, aims to optimise the management of the project (with) first fuel loading and start-up of the reactor in the fourth quarter of 2018”, it said. “The deal must represent value for money and is subject to approval by ministers”.

Coyne further urged energy secretary Amber Rudd to use her ministerial position to press potential investors for a quick decision on the necessary future investment, so that EDF can make a final investment decision before Christmas.

The repeated delays and cost overruns at Flamanville and at another EPR being built in Olkiluoto, Finland have weighed heavily on the French nuclear industry’s reputation and helped competitors from South Korea and Russian Federation win market share.

Costs of nuclear are hard to compare from one country to another, but the gulf between projected costs in China and the United Kingdom is indisputable.

Levy put China’s racing ahead to be first with an EPR reactor down to differences in nuclear security regulations as well as building and labour practices. But it is also partly because state-run nuclear enterprises can borrow at very low or even zero rates of interest.

Hinkley Point in the English county of Somerset.

Costs for the joint venture with atomic energy giant Areva have now soared to €10.5bn (£7.7bn).

But the discovery of weak spots in the reactor vessel and problems with safety valves have added to doubts about the EPR, which France and Britain have chosen to replace their older reactors, a costly process that will take decades to implement.