LESS than three years ago the British government struck a deal with EDF, a French state-owned utility, to subsidise the first new nuclear power station built in Britain since 1995: Hinkley Point C on the Somerset coast. The agreement was hailed by David Cameron, the then-prime minister, as “brilliant news”. But a lot has changed since then—and not just the incumbent at 10 Downing Street.

On July 28th, hours after EDF’s board narrowly endorsed a decision to go ahead with the £18 billion ($24 billion) Hinkley Point investment, the new government of Theresa May unexpectedly slammed the brakes on, launching a review of the project that it says it will finish by the autumn. It is understood to want to probe a deal with China General Nuclear Power, a Chinese state behemoth, which had offered to stump up one-third of the price tag in exchange for permission to build a nuclear-power station of its own at Bradwell, in Essex. The delay is the clearest sign that Mrs May is rethinking the open-door industrial policies of her predecessor (see article).

Yet analysts say there is more to the delay than mere Sinophobia. Hinkley is “big and based on last-century technology, which is not what the UK’s power system needs for the future,” says Michael Grubb of University College London. A review of the assumptions prevailing when the government struck the deal reveals how flimsy the economic rationale was. In 2012 Britain’s energy boffins predicted that for the foreseeable future the price of non-nuclear fuels, such as natural gas, would be more than double where they are today. As a result, they estimated that wholesale electricity prices—the basis for determining the level of subsidy to EDF—would remain above £70 per megawatt hour. They are currently below £40. Last month the National Audit Office, a spending watchdog, said that forecasting error alone had almost quintupled the implied value of the subsidy, from £6 billion to almost £30 billion over 35 years.

At the time, the civil servants reckoned that by 2025, when Hinkley Point is due to open, the cost of producing electricity from a nuclear-power station would be lower than from a gas-fired one—and much lower than from wind farms and solar-power plants. They have since reversed those views (see chart). Since Hinkley became a serious proposal less than a decade ago, the cost of nuclear power has increased, that of renewables has fallen and the price of battery storage—which could one day disrupt the entire power system—has plummeted. What is more, EDF’s nuclear technology has failed to get off the ground in the two projects in Finland and France that have sought to use it. “When so much has changed, it would have been inappropriate not to pause,” says Professor Grubb.