There are calls for an inquiry into tests at Volkswagen which reportedly involved exposing monkeys to toxic diesel fumes.
Reports have emerged that three major German carmakers financed research which used monkeys to test the health effects of diesel exhaust.
The New York Times and a new Netflix documentary series, Dirty Money, revealed details of the research reportedly conducted by Volkswagen, Daimler and BMW.
The reports say that car giant Volkswagen worked with European researchers to carry out the tests.
One member of the company’s supervisory board says the experiments were "absurd and inexcusable".
The experiments, said to have been carried out in 2014, involved a group of monkeys being exposed to exhaust from a late-model diesel Volkswagen while another group was exposed to exhaust from an older Ford diesel vehicle for four hours.
Following this period of exposure, researchers took samples of lung tissues to test for inflammation.
The experiments did not kill the monkeys but it remains unclear what effect the exposure to exhaust had on them following the research.
The research was commissioned by the European Research Group on Environment and Health in the Transport Sector and the experiments were carried out in the Lovelace Respiratory Research Institute in Albuquerque in the US.
A statement released by Volkswagen said: "The scientific methods used to conduct the study were wrong.
"Animal testing is completely inconsistent with our corporate standards.
"We apologise for the inappropriate behaviour that occurred and for the poor judgement of individuals who were involved."
Daimler said in their statement that they have launched an investigation into the study: "We will clarify how the L.R.R.I. study came about and have launched an investigation.
"Daimler does neither tolerate nor support unethical treatment of animals.
"The animal experiments in the study are superfluous and repulsive."
Yesterday, BMW released a statement clarifying that the company "does not carry out any animal experiments".
It stated: "The BMW Group did not participate in the mentioned study and distances itself from this study."