Hybrid cars may not be as green and clean as was previously thought.
That's according to a new report by the European climate NGO Transport and Environment, which has cast doubt on their CO2 emissions.
It found that they could emit as much as eight times more carbon dioxide than previously thought.
There are fears that this could be the follow-up to the diesel-gate car scandal.
Julia Poliscanova, Senior Director at Transport and Environment, said: "Even when they are driving in what carmakers call zero-emission mode, they are not.
Poliscanova said they are only efficient if you drive them softly.
"So basically, a prime example is you think you're driving in a zero-emission electric mode, but you put the heating on, or you put your lights on, or you accelerate, that's it!"
She added that governments should not be subsidising them.
The European Automobile Manufacturers Association has refuted the allegations in the report.
It said: "We question the methodology of this study which analyses vehicles over long trips only."