The results of rigged animal tests that have plunged the German car industry into renewed turmoil were suppressed because they showed the opposite of what car makers wanted, it has emerged.

Monkeys were forced to breathe diesel exhaust fumes from a VW Beetle for eight hours in an attempt to prove they were not toxic in a study funded by Volkswagen, BMW and Daimler, the company that makes Mercedes.

But it has now emerged that the experiment found the exhaust was more harmful than fumes from a 20-year-old diesel car despite the fact the Beetle was fitted with software designed to lower emissions during tests.

The car makers allegedly refused to pay the final bill for the tests, which were conducted in the US, after the results were not what they were expecting.

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The details emerged as senior managers were suspended at BMW and Daimler in the wake of the scandal, in a sign the crisis is spreading beyond VW, the only company to admit responsibility so far.

The US study's findings were not released at the time but were made public on Tuesday in details leaked to Germany's Bild newspaper.

Also published were emails from the scientists which clearly show they were concerned the car makers would not be happy with the results.

"I'm a little uncomfortable sending it out without approval. The findings are obviously not what was expected and I'm not sure how they want to proceed," reads an email from Jacob Mcdonald, the research director.

An email from Jeremy Brower, another member of the team, says: "Here is a draft of the report. Please review at least the conclusions before sending it out. I was trying to soften the blow of the results from the study without saying it was a bad study."

In the tests, monkeys were forced to inhale exhaust fumes from the VW Beetle for more than eight hours and were then examined for harmful effects.

Monkeys were forced to breathe diesel exhaust fumes for 8 hours to prove they weren't toxic. Photo / AP
Monkeys were forced to breathe diesel exhaust fumes for 8 hours to prove they weren't toxic. Photo / AP

A second control group of monkeys were made to inhale fumes from a 1997 diesel Ford pickup.

But to their surprise the scientists found the monkeys exposed to the exhaust from the new VW suffered more inflammation of their internal organs than those to the old Ford.

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The tests, carried out by the Lovelace Respiratory Research Institute in New Mexico, were commissioned by EUGT, a lobbying group set up and funded by VW, BMW, Daimler and German engineering giant Bosch.

Bosch ended its participation in the group in 2013, a year before the tests took place and denied any involvement.

EUGT paid US$649,000 in advance for the study, according to Bild.

But the newspaper alleged the lobby group refused to pay an outstanding US$71,857 because it was unhappy with the results.

Until now VW has borne the brunt of the scandal because it allegedly took the lead in commissioning the study, which involved a VW car.

But the other two carmakers moved to limit the damage yesterday.

Udo Hartmann, the head of environmental protection at Daimler, and Frank Hansen, BMW's head of urban mobility, were both suspended pending internal investigations.

VW suspended its chief lobbyist on Tuesday.

BMW has denied any involvement in the study, and Daimler has publicly distanced itself.

But yesterday's move by the car makers came after EUGT's chief scientific adviser said their representatives on the lobby group's board were fully aware of the study.

"Of course the car manufacturers knew about the tests on monkeys," Prof Helmut Greim said.

"The current attitude of the carmakers' boards doesn't make sense to me."

Prof Greim defended the tests.

"It may sound bizarre to laymen, but studies on monkeys are done everywhere, and for certain questions such studies are very significant," he said.

This article was originally on the Daily Telegraph and is republished with permission.