Wealthy businesswoman Johanna Quandt, her son Stefan Quandt and daughter Susanne Klatten, who together hold a 46.7 percent stake in the luxury car maker, made the donation of 2,30,000 Euros each to the CDU on October 9, two weeks after the German elections, according to parliamentary records.
Under pressure from the German government, the EU environment ministers on Monday agreed to delay the introduction of emission limit of 95 grams per kilometre for average cars until 2024 during their meeting at Luxembourg.
The ministers have agreed in July that the new regulation should come into effect in 2020.
German Environment Minister Peter Altmeier, however, said that the decision to revise the earlier deal was intended to combine "clarity in climate protection with the necessary flexibility and competitiveness to protect the auto industry in Europe".
Opposition parties, environmentalists and anti-corruption organizations have charged that the German government's intervention to delay the EU legislation was influenced by the USD 9, 32,000 donations.
"There is a clear link between the donation and Peter Altmeier voting against tightening the EU regulations on CO2 emissions for cars from 2020," Left party's Gregor Gysi said.
"We must stop this practice of industry attempting to influence government policies once and for all. Otherwise we will lose our credibility," Gysi added.
Social Democratic Party's (SPD) deputy parliamentary leader Ulrich Kelber urged the CDU to return the donation "in the interest of its political credibility".
The Green party, which on Tuesday abandoned talks with the CDU on forming a new coalition, also criticized the donation. The party's environment policy spokesperson Baerbel Hoehn said a link between the donation and the German government's intervention cannot be overlooked.
The CDU and the Quandt family have, however, strongly rejected these allegations. The Quandt family said in a statement that it has been supporting the CDU with private donations for several years regardless of whether it is in power or in the opposition.
Amnesty International chairperson Edda Mueller urged German president Joachim Gauck to set up a commission to reform the present system of financing political parties.
The controversy comes ahead of Merkel's conservative party's third round of talks with the SPD later this week to explore the possibility of forming a coalition government.
Merkel's party emerged as the dominant force from the election, but with 311 of the 631 seats in the German Parliament, the Bundestag, they lack a majority.


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