A French government report omitted significant details of how Renault’s diesel cars were able to emit fewer deadly gasses when subject to official emissions testing, members of the state inquiry have said.

The inquiry’s report, published last month, found some models emitted nitrogen oxides – a cause of respiratory diseases linked to early death – at levels nine to 11 times higher than EU limits.

But three of the commission's 17 members said the published report did not include the full details of their findings, including the fact that a NOx "trap" in the Renault Captur went into overdrive when the sport utility vehicle was prepared for emissions testing but not during normal driving conditions.

It was discrepancies between on-road and in-laboratory NOx emissions that first tipped off US authorities that Volkswagen diesels had been installed with illegal “defeat devices” that cheated tests, triggering billions in fines and a criminal investigation.

The French commission members said they had no evidence that Renault was using similar devices, arguing only that further investigations were needed to find out why the cars performed differently during testing. But the omissions have led members to fear the government may be too lenient because of its 20 per cent stake in the carmaker.

"The report was ultimately written by the state and they decided what would remain confidential," said Charlotte Lepitre, an executive of France Nature Environment, the umbrella group of French environmental groups, who sat on the commission.

Renault denied using software to cheat emissions testing, saying its models “conform to the laws and norms in each market where they are sold”. Renault recalled 15,000 vehicles in January and has agreed to implement more robust de-polluting technologies as part of a wider “action plan”.

A French environment ministry official denied any intention to hide facts, saying the government incorporated the commission’s feedback when it consolidated comments from its various members. France’s independent fraud authority is also continuing an investigation into the carmaker’s practices.

But a government official acknowledged the state’s special relationship with Renault, saying: “The government is sensitive to the brand image of the companies it has invested in.”

– Copyright The Financial Times Limited 2016