German giant Volkswagen's worldwide pollution cheating scandal threatens to backfire on diesel, the fuel that powers most new cars in Europe and is defended by manufacturers as a vital means to curb global warming.
Diesel spews out less heat-trapping carbon dioxide than gasoline, the industry argues.
But without sophisticated controls, the fuel may also emit harmful levels of poisonous nitrogen oxides and lung-clogging particles.
Revelations that Volkswagen equipped 11 million of its diesel cars worldwide with software that can switch off those pollution controls -- except when it detects it is undergoing official testing -- have sharpened the focus on diesel's risks.
Automobile manufacturers were reluctant to comment on diesel's attributes in the midst of the Volkswagen scandal, which cost the group's chief executive Martin Winterkorn his job on Wednesday.
But former Renault boss Louis Schweitzer laid bare the worst fears of major manufacturers on Thursday when he told French business daily Les Echos: "It would be disastrous to conclude that diesel must die."
For some politicians, that is precisely the lesson to be learnt.
"Let's stop lying to the French people by encouraging them to buy so-called ecological cars... clean diesel does not exist," said Emmanuelle Cosse, French national secretary of Europe Ecology - The Greens.
In France, where most drivers use diesel, 59.6 percent of people say they do not believe the fuel is clean, according to a Harris Interactive survey published this week.
A separate survey by Tilder/LCI/Opinionway asked 1,000 people if the Volkswagen revelations had changed their view of diesel cars: 27 percent said they now had a "more negative" opinion while 70 percent said it made no difference.
"This scandal may yet be seen as a turning point in automotive history. Hybrid and all-electric technologies, in which Apple and Google are poised to pounce, could be the beneficiaries," the Times of London said in an editorial on Wednesday.
- 'Diesel in the firing line' -
"Diesel is in the firing line of the authorities and some ecological activists, and the Volkswagen case is not going to help things, it could make them worse," said Meissa Tall, automobile industry analyst at management consultancy group Kurt Salmon.
Lawmakers will have a significant influence on whether people choose gasoline or diesel engines because subsidies for diesel have greatly helped its popularity, she said.
The Volkswagen scandal erupted three weeks after the launch of new European anti-pollution standards known as Euro 6, which introduced stricter limits on NOx, or nitrogen oxides: nitrogen monoxide and nitrogen dioxide.
Carmakers in Europe argue that diesel cars are essential to achieving a mandatory European Union target of reducing average carbon dioxide emissions from new passenger cars to 95 grams per kilometre by 2020.
"Any sign of a political desire to remove diesel from the options available to achieve the average CO2 targets will be to the detriment of manufacturers' ability to reach the targets," Renault chief executive Carlos Ghosn, who is also the current head of the European Automobile Manufacturers Association, said in June.
Putting old-style diesel engines in the same basket as the new Euro 6 compliant models would be "unfair" considering the efforts carmakers have expended to make their vehicles less polluting, he said.
Even before the Volkswagen scandal, however, diesel was beginning to wane in Europe, whose drivers are by far the world's largest users of the fuel.
Diesel powered 53 percent of new cars sold in the European Union in 2014, down from 55.2 percent in 2011, the carmakers' association said.
In France, where diesel is particularly popular, the share dropped to 58 percent this year from 77.3 percent in 2008, largely because the new regulations led manufacturers to abandon diesel for smaller models.
Manufacturers say the real problem is not new vehicles but the older automobiles already on the road in the European Union. Of the more than 250 million vehicles in the bloc, the average age is nearly 10 years, the carmakers' association says, and 41 percent of them are running on diesel.