Germany blocked the introduction of tougher EU emissions rules for cars shortly after Chancellor Angela Merkel's party received a large donation from three major BMW shareholders.
The opposition Left Party cited the donation as evidence of an uncomfortably close relationship between Mrs Merkel and German carmakers.
At Germany's insistence, EU environment ministers delayed agreement on the proposed emissions rules scheduled to come into force in 2020.
Mrs Merkel's Christian Democratic Union said the 690,000 euros (£584,000) last week from Susanne Klatten, her mother Johanna Quandt and brother Stefan Quandt was "completely unrelated to any individual political decision."
The Quandt family holds almost half the shares in Munich-based BMW, whose cars on average emit well over the proposed limit of 95 grams of carbon dioxide per kilometre.