Germany's main centre-left party has voted overwhelmingly to go into government with Chancellor Angela Merkel's conservative Christian Democrats, clearing the way for Merkel to start her third term at the head of a new coalition.
The Social Democrats' (SPD) vote on Saturday means the new government can take office on Tuesday.
The SPD said that 76 percent of its grassroots members who took part in a postal ballot voted in a referendum in favour of joining a right-left coalition with the conservatives.
The ballot of the Social Democrats' nearly 475,000 members caps post-World War II Germany's longest effort to form a government.
Party treasurer Barbara Hendricks said a resounding 76 percent backed a left-right coalition pact after a postal ballot among its 475,000 members.
The unprecedented, binding referendum, which drew nearly 78 percent turnout, marked an unqualified triumph for SPD chief Sigmar Gabriel, nearly three months after his party's bruising loss to Merkel's Christian Democrats (CDU) in the general election.
"I have never seen my party so politically engaged in the 36 years I have belonged to it," a beaming Gabriel said, flanked by the party leadership.
"It's been a long time since I was so proud to be a Social Democrat."
Merkel's Christian Democrats (CDU) and their Bavarian sister party, the Christian Social Union (CSU), won the September 22 election but fell short of a majority.
They had spent much of the past three months negotiating a coalition agreement with the SPD, a distant second in September.
The leaders of the three parties will announce the 15 members of the cabinet on Sunday.
The coalition agreement was due to be signed on Monday and Merkel's new government could be sworn into office on Tuesday, making her only the third chancellor in German post-war history to serve a third term.
Finance Minister Wolfgang Schaeuble, one of the main architects of Germany's tough-love response to the eurozone crisis, would stay on, according to media leaks.
Meanwhile Gabriel was to head up a "super-ministry" in charge of the economy and Germany's ambitious energy transformation away from nuclear power and toward renewables.
Germany appeared poised to tap its first woman as defence minister, Ursula von der Leyen, a mother-of-seven.
Gabriel called the rank-and-file vote to rally his often fractious party around the left-right government and drove a hard bargain during the coalition talks, which wrapped up in late November.
He extracted concessions including Germany's first national minimum wage, permission for dual citizenship for children of immigrants, restrictions on temporary jobs and a lowering of the retirement age to 63 for those who paid into the system for 45 years.