Social Democratic Party leader Martin Schulz is likely to succeed German Foreign Minister Sigmar Gabriel, following the announcement of a coalition deal by SPD with Chancellor Angela Merkel's CDU/CSU bloc.
Chancellor Angela Merkel's conservatives and Germany's main center-left party are still haggling over a deal to form a new coalition government after negotiations that lasted through the night.
Schulz had originally said he would not take a job in a government led by Merkel but then said on Wednesday he wanted to take the helm at the foreign ministry - a move for which he was widely criticized.
The two men are former friends and Mr Gabriel has alleged they made a deal in 2016 under which he would get to stay on as foreign minister in return for stepping aside as party leader to make way for Mr Schulz. He was officially elected by a party congress with 100 percent of the vote.
Prospective German Foreign Minister Martin Schulz has announced that he will not take up a ministerial post in the next coalition government.
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"I hereby forgo joining the federal government", Schulz announced, adding, "at the same time, I sincerely hope that this will end the personnel debates within the SPD". Schulz had suggested his successor should be parliamentary group chief Andrea Nahles, according to German media.
The coalition agreement "certainly won't simply be waved through", CDU MP Sylvia Pantel told mass-market daily Bild.
"We shouldn't only talk about how we want to shape the next four years in Germany but also what the CDU will stand for in future, which topics we can win elections with in the next 10 years and people go along with topics", he said.
The SPD's traditional left-wing youth organization, Jusos, have opposed a coalition government with the Christian Democrats, and called for reform within the party.
Merkel, 63, who long seemed immune to criticism from within her own ranks, came under fire this week after agreeing to have just six ministers in the cabinet - the same number as the SPD, which won fewer votes in the election.