Developer EDF has announced fresh delays in the construction of its prototype EPR at Flamanville, France, caused by the discover of defective welding.

EDF said on 25 July that first power generation at Flamanville would now slip by a year to early 2020 because it needed to repair “quality deficiencies” in the welding in conduits carrying steam to the turbines. The cost of the plant has increased by a further €400 million to €10.9 billion, more than three times its original budget.

City analysts at RBC Capital Markets said the announcement would “add to concerns about whether EDF’s other projects . . . can be delivered on time and budget”.

This is a reference to the 3.2 GW Hinkley Point NPP complex, currently in the early stages of construction. UK prime minister Theresa May gave the plant the go-ahead in 2016 despite widespread concerns over high subsidies to be paid by consumers and about EDF’s inability to build reactors on time and to budget.

Hinkley Point had already been delayed from its original 2007 plan to start generating by end-2017. Costs had risen to £18 billion by the time it got the go-ahead. EDF raised the estimate to £19.6 billion a year ago and warned that start-up could be delayed to 2027 but has since insisted it is sticking to the 2025 start date.

EDF began working on the EPR (European Pressurised Reactor) type, 25 years ago. It was originally due to start operating in 2012. Four reactors were scheduled to be operating by now – in France, Finland and China – but construction has been plagued by problems and only one, in Taishan, southern China, is operating.

The most serious issue delaying Flamanville to date was the earlier discovery of a weakness in the reactor vessel. The French factory that made the vessel was subsequently found to have falsified safety tests for components supplied to the French nuclear industry.

EDF insists it has learnt the lessons from the EPRs being built elsewhere, ensuring that the British project will proceed more smoothly. However, the UK’s nuclear safety regulator the Office for Nuclear Regulation has raised concerns about substandard quality control checks on EDF’s supply chain.

Kate Blagojevic, head of energy at Greenpeace UK, commented: “EDF’s nuclear design just doesn’t work very well. The nuclear power plant in Finland [Olkiluoto] is a decade late and because of yet more technical problems, the Flamanville plant has gone from late to later. This bodes ill for Hinkley Point C.”

A spokesman for EDF said: “The construction of Hinkley Point C remains on track. The project has already benefited, and will continue to learn from the experience of other projects.”