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.
The politician said she would resign as head of the SPD on Monday and quit as leader of the party in parliament on Tuesday.
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 criticised 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.
"Three grand coalitions in a row would not do democracy in Germany any good," he was quoted as saying.
The coalition is due for a midterm review in autumn, which could be an opportunities for the SPD to pull the plug on the ruling alliance.
The SPD is now scrambling to put an end to infighting to stabilise its leadership.
A Forsa poll from Saturday showed that Scholz is more popular than Chancellor Merkel's heir apparent, Annegret Kramp-Karrenbauer.
Last week German weekly Bild am Sonntag reported that former SPD leader Martin Schulz wants to replace Nahles as head of the parliamentary party.
Former SPD leader Martin Schulz told Germany's Welt am Sonntag newspaper that he would not stand against Nahles in the leadership vote on Tuesday but declined to answer whether he would seek to be re-elected leader thereafter.
