On granite cliffs overlooking the Channel is France’s most famous building site. If all goes to plan, by the end of the decade this rocky outcrop will house the biggest and most powerful nuclear reactor in the world.

The technology behind the European pressurised reactor (EPR) is meant to be safer than anything that has gone before. But the project is more than three times over budget and years behind schedule, and France’s nuclear safety authority has found weaknesses in the reactor’s steel.

And the same model could soon be coming to the English coastline at an even bigger cost. France’s state-controlled energy giant, EDF, is expected to announce on Thursday whether it will go ahead with its investment into the £18bn Hinkley Point C power station in Somerset, where two EPR-style reactors are proposed. EDF is expected to come out in favour of the massive project, despite strong opposition from trade unionist board members, who argue the French government cannot afford it.

Perhaps not since the D-Day landings will the British government have taken such a keen interest in the Normandy coast. If and when it comes online, possibly in late 2018, the Flamanville EPR will be the world’s largest nuclear reactor. It will also be more efficient, using less fuel and producing almost a third less waste than older reactors, according to the technical specifics provided by EDF. The reinforced concrete core is intended to withstand plane crashes and earth tremors. But the combination of the EPR’s size and its safety features have turned it into a construction nightmare.

Today, not a single EPR reactor operates anywhere in the world. In Flamanville, the first concrete was poured in 2007. Since then costs have more than tripled to €10.5bn, while the project is six years behind schedule. In Finland, the location of another EPR, the picture is even worse: the Olkiluoto reactor is nearly a decade behind and three times over budget, with the added headache of legal battles over who is to blame. Less is known about two EPR reactors being built in China.

What might have been just a colourful tale of shareholder angst took a different turn in 2015, when it emerged that weak spots had been found in the Flamanville reactor’s steel, which is made by another French industrial champion, Areva. France’s Nuclear Safety Authority (ASN) said it had found “very serious anomalies” in the reactor vessel. As the regulator deepened its investigation, it warned that the problems could affect other reactors in operation, although it stressed that France’s reactors were safe. In its latest annual report, the ASN described the safety of the country’s 58 nuclear power plants as “satisfactory”, but said it had “significant concerns” for the future as financial pressures build on France’s nuclear industry.

Even before the 2011 Fukushima disaster turned some countries off nuclear power, the horizon for France’s nuclear industry was clouded. It has made a dash for big, costly projects, just as electricity production is moving to a smaller, more dispersed model with the arrival of renewables. Meanwhile, demand for power has been falling in continental Europe as a result of factory efficiency drives.

Martin Young, an analyst at RBC Capital Markets, questions whether anyone needs to be building nuclear power stations on the scale of Flamanville or Hinkley Point C. “We should probably think about smaller, easier-to-build, more-flexible nuclear power stations,” he says.

Instead, the EPR prestige project has forced Areva to the brink of bankruptcy. The reactor builder is cutting 1,000 jobs and has been forced into a shotgun merger with EDF at the behest of the French government. No wonder the business newspaper Les Echos has come up with a new name for the EPR: Enormous Problems to Resolve.

The EPR woes have raised the stakes over Hinkley Point C, whose two reactors are intended to supply 7% of British electricity by 2025. But the British project, to be co-funded with China, has become another enormous headache. In March, EDF’s finance director, Thomas Piquemal, resigned because he felt his warnings that Hinkley Point C could bankrupt the company were being ignored.

France’s economy minister, Emmanuel Macron, argues that Hinkley Point C is vital to secure EDF’s position in the UK, but also to promote “our expertise in this major technology around the world”. In May, he told Le Journal du Dimanche that if Hinkley Point C did not go ahead, “we would give up our place to our American, Chinese or Russian competitors”. He also questioned where France would find the skills to renew its own ageing nuclear power stations.

Some argue that savoir-faire in the French nuclear industry has already begun to wither. Mycle Schneider, a Paris-based nuclear policy analyst, accuses the industry of overestimating its capacity to build highly complex reactors, while underestimating skills gaps. “It is clear that the skills issue is part of the problem. Areva hasn’t built reactors [in France] for a very long time,” he says.

Schneider is worried that relentless cost-cutting pressures could compromise safety, as Areva bids to save €1bn by 2017, through job cuts. “To me, it is very obvious that you will cut into safety and security and that is what makes me most nervous,” he says. “The financial and economic pressure on all the stakeholders is completely unparalleled.”

The decision on Hinkley Point C may come down to politics. Young at RBC suggests that EDF wants a decision on Hinkley Point C while the UK has a functioning government and before possible Brexit aftershocks complicate the picture. “If you are EDF, why would you want to wait and run the risk of a snap election being called?” he says. A future government could change its mind on the project, Whitehall’s official spending watchdog has said, warning of a “tidal wave” of pressures from an impending Brexit.

Patrick Fauchon, the mayor of Flamanville, is no stranger to political pressures. He recalls strong tensions in the 1970s when locals first debated plans to build two reactors. He has been in post since 1983, two years before the first Flamanville reactor went into service, followed by the second a year later.

These reactors were among the 58 built by France in a rapid sprint between 1977 and 1999, spurred into action by the oil price shock. Like high-speed trains and Concorde, nuclear energy was viewed as a symbol of progress. For small towns such as Flamanville, it was a welcome source of jobs. The nuclear industry is by far the biggest employer in the region, estimated to provide one in two local jobs.

The nuclear power station took over the site once occupied by the local granite mine, which closed in 1962. Once, Flamanville shipped granite all over France, including that used in Paris’s Place de la Concorde. Now it supplies nearly 5% of the country’s electricity. “There is a continuity of industrial life here,” says Fauchon.

There are dissenting voices. A poster advertising an anti-nuclear rally along the tricolour-festooned high street underlines that not everyone has fallen head over heels for nuclear. But most people are content to support the industry. “The locals are very happy to take advantage of the huge financial windfall provided by La Hague [a nuclear fuel reprocessing plant] and Flamanville,” says Jacky Lebuhotel, a reporter. “Delays to the EPR may raise an eyebrow, but they are also a source of significant financial return.”

Among the neat little yachts and cheerful summer blooms in the harbour at Diélette, it is easy to forget the nuclear plants that surround the bay – Flamanville on one side, La Hague on the other. Several people said they gave no or little thought to the local nuclear industry. Only the steady stream of trucks trundling past the tidy granite houses give a hint of a grittier business.

Unsurprisingly, nuclear workers take a close interest in the EPR’s fortunes. Philippe Revel, a 51-year old foreman, is not convinced that the much vaunted next-generation nuclear model is an improvement on its “simpler, more functional” predecessors, or that it will play a big role in France’s nuclear future.

Asked whether he would recommend it to the British, he smiles wryly. “Personally, no. It is too complicated. It is too big.”

• This piece was amended on Thursday 28 July 2016 to make clear that Mycle Schneider was referring to Areva’s reactor-building experience in France.