The German magazine Auto Bild reported that BMW's X3 crossover emitted nitrogen oxides at levels 11 times higher than allowable limits according to findings by the International Council on Clean Transportation.
The ICCT previously helped trigger a probe of Volkswagen's diesel emissions levels in the U.S.
BMW, however, strongly denied the report and said that emissions do not vary "whether they are on (test) rollers or on the road," Reuters reported.
"No specific details of the test have yet been provided and therefore we cannot explain these results," the company added. "We will contact the ICCT and ask for clarification of the test they carried out."
BMW's stock nonetheless fell 7 percent in early trading.