A Swiss study claims to have found that Renault’s Espace minivan series releases toxic diesel emissions 25 times higher than the legal limit.
In a finding that echoes the recent 'VWgate' scandal, the Espace was said to be releasing the emissions despite complying with EU tests.
DUH group, which commissioned the tests, believes that official tests may have been carried out with the engine operating at unrealistically low temperatures.
In a statement, Renault said it contested the findings of the DUH group.
When run with a warm or hot engine, a 1.6-litre Espace of the latest Euro 6 diesel generation emitted up to 2.06 grammes of NOx per kilometre, the campaign group said.
This is more than 25 times the EU limit; the vehicle was found to have met the statutory 80 milligram cap only with a cold engine after "specific pre-conditioning".
"It's unbelievable that so-called modern diesel vehicles that damage the air we breathe in this way are on the road today," campaigner Axel Friedrich said in the DUH statement.
He said Europe needs a "comprehensive reorganisation of the system in which mandatory regular controls on the street are integrated".
The VW diesel scandal has drawn attention to a wider pattern of legal test manipulation that stops short of outright cheating.
The EU rules themselves are now acknowledged to be inadequate even by manufacturers such as Peugeot.
Carmakers were found to be routinely stripping out standard equipment to reduce test vehicles' mass, taping up door joints and fitting bald tyres that would be illegal on the road.
Attempts by EU regulatory bodies to to phase in real-world emissions measurements were watered down in committee last month under sustained German-led lobbying.
As well as taking a sizable hit to its finances, VW has admitted that three million cars in Europe equipped with 1.6 litre diesel engines will require hardware changes to disable the emission-cheating devices which were installed as part of their system.