Germany's Social Democratic Party (SPD) chief Andrea Nahles said she would resign on Sunday as well giving up her position as head of its parliamentary group, saying she did not have the support she needed to lead it.

The resignation casts fresh doubt over the durability of Germany's ruling coalition, which came close to falling apart last year. The country's mainstream Christian Democrats and Social Democrats suffered heavy defeats in last week's European elections.

Germany's Greens have overtaken Chancellor Angela Merkel's conservatives to become the most popular party in Germany, an opinion poll on Saturday showed, with the SPD support hitting an all-time low.

"The discussions within the parliamentary faction and feedback from within the party have shown me that I no longer have the necessary support to carry out my duties," Nahles said in a statement released by the SPD on Sunday.

Nahles was due to face a vote on her leadership position on Tuesday after her decision to stay in coalition with Angela Merkel's conservatives was criticized by the party's left.

Earlier, German vice chancellor Olaf Scholz told Germany's Tagesspiegel that he had ruled out entering another grand coalition as the country's Social Democrats seek to regroup after losses during the European parliamentary elections last week.

"I am very sure that it would not be justifiable for us to have a fifth grand coalition," Scholz told the German paper in an interview published on Sunday before Nahles announced she would resign.