SPD parliamentary group leader Andrea Nahles told German radio today that she would join the meeting along with SPD leader Martin Schulz, who has called for a United States of Europe "by 2025".
The head of Merkel's Bavarian sister party, Horst Seehofer, will also attend the talks along with Nahles' conservative counterparts in parliament.
At a fractious party congress yesterday, Schulz won members' approval for "open-ended" talks that could lead to either a new right-left "grand coalition", a Merkel minority government or, if negotiations fail, new elections.
Schulz, the former president of the European Parliament, vowed to extract a high price if the SPD supports Merkel's bid to lead Europe's biggest economy for a fourth term from early 2018.
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Schulz was later reelected party chief with almost 82 per cent of the vote.
The SPD motion said the party was "aware of its responsibility to our country" and "not indifferent" to whether a new government is formed, but pledged that any government that emerges must drive forward "the fundamental renewal of Europe".
"You don't go into talks with a giant rucksack full of red lines - otherwise you can forget about negotiations.
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