Prosecutors in Paris have opened an inquiry into allegations that a factory owned by Areva, the French nuclear group, has been falsifying safety tests for decades.

The factory has already made one key component for the reactors planned at Hinkley Point in Somerset but this was scrapped amid safety fears and a replacement was ordered from Japan. The Hinkley Point reactors were designed by Areva and are being built by Électricité de France (EDF), the state-owned company.

EDF is poised to take over the factory at the centre of the inquiry, which may be asked to make further components for the Somerset reactors, a Paris source claimed. The Hinkley Point project, in which EDF has a two-thirds stake and China General Nuclear Power Group the remaining third, is designed to produce seven per cent of Britain’s electricity.

Critics say that the criminal investigation raises searching questions about the trustworthiness of the French engineers behind the scheme.

“From a British background, it is inconceivable that nuclear safety documents should be falsified,” said Paul Dorfman, honorary senior research fellow at University College London’s Energy Institute.

“How can one be assured of the quality of key nuclear materials given the fact that the French have been falsifying documents and installing faulty equipment that is key to nuclear safety?”

The investigation comes after the discovery of a flaw in a 116-tonne reactor pressure vessel installed in a project at Flamanville in Normandy.

The French Nuclear Safety Authority has ordered tests to determine whether the component could crack and cause a nuclear accident. The watchdog says that it will decide next year whether to allow the plant to open. Prosecutors were told of the flaw, which involves an unexpectedly high level of carbon in the steel.

Prosecutors in Paris said that a criminal investigation had been launched into claims that the Areva factory at Le Creusot in Burgundy, which made the component, deliberately faked the safety tests.

The factory made a reactor vessel head of the same kind for Hinkley Point. This has had to be cut up and subjected to checks to determine the extent of the risks caused by the excess carbon.

EDF said it was confident that its Normandy plant would get the green light. After discovering the problem at Flamanville, an inquiry was launched at Le Creusot factory, which revealed evidence that safety tests had been falsified over the past 60 years. The investigation initially concerned 400 files but was subsequently extended to 9,000.

France’s nuclear watchdog said that 87 “irregularities” had been detected so far, either in nuclear components or in the casing used for their transportation.

Inspectors said that documents stating that the components were safe were based on the wrong figures.

Prosecutors believe that the irregularities might have resulted from a deliberate attempt to cover up a safety risk. About 20 of the irregularities involved components intended to be used in the new reactor in Flamanville. The rest were proposed for reactors already operating in France. A further inspection by the nuclear watchdog found that components made in Japan had the same problem of excess carbon.

Pierre-Franck Chevet, chairman of the European Nuclear Safety Regulators Group, said: “We are facing a serious anomaly.”

EDF was ordered to undertake emergency tests on 18 of its 58 French reactors to determine whether they were safe. All but four have now been given permission to re-start.