The move comes amid rising public concern about potential attempts by Russia to interfere in elections on September 24 in which German Chancellor Angela Merkel is running for another term. Rosneft is the target of Western sanctions over Moscow's aggression in Ukraine, according to RFE/RL.

Schroeder, who has called Russian President Vladimir Putin his close friend, defended his decision on August 17 and said it would not damage his Social Democratic Party, where some members have distanced themselves from him over the matter.

Read alsoNaftogaz: No plans to give up claims against Gazprom"I don't think my post will have a negative impact on the election. Germans have a big interest in having sensible ties with Russia," he told Swiss newspaper Blick in an interview.

"This is a political campaign for Mrs Merkel. People want to help her by slandering me," he said. "I would like to help improve relations between Russia and the EU."

Read alsoGermany's ex-Chancellor Schroeder gets top position at Gazprom subsidiarySchroeder, who was chancellor from 1998 to 2005, said he knew his decision would cause a stir but he had not expected such "one-sidedness" in media coverage.