The members of Germany's Social Democrats have voted to back a new grand coalition with Chancellor Angelo Merkel, according to media reports.

The Social Democrat (SPD) party's 464,000 members were asked to vote on a proposed new coalition with Chancellor Angela Merkel's conservatives, opening the way to a new government for Europe's economic powerhouse.

Five months after an inconclusive election, and after the failure of Merkel's first attempt to form a government with two smaller parties, the long-serving chancellor's best hope of securing a fourth term was with the SPD.

Volunteers began counting the postal ballots at 10 p.m. and worked through the night.

A "No" from a membership that is wary of repeating the bruising experience of the past four years of "grand coalition" would have pitched Germany into greater uncertainty at a time when the European Union is looking to its largest country for leadership on a host of economic and security issues.

SPD acting leader Olaf Scholz said on Saturday turn-out in the poll had been "very, very high" after an intense internal campaign that pitted the party's pro-coalition leadership against its more radical youth wing, which campaigned for "No".

The SPD initially planned to go into opposition after a disastrous result in September's election, but agreed to negotiate with Merkel's conservatives after talks with the pro-business Free Democrats (FDP) and the environmentalist Greens on a three-way tie-up collapsed in November.

They thrashed out a coalition agreement which SPD leaders hailed for its commitments to strengthening the EU and giving them some key government roles. But opponents say the SPD risked irrelevance if it plays second fiddle to Merkel again.

A poll published on Sunday showed the SPD falling one percentage point to 16 percent, its lowest ever reading in the Emnid survey. That means it has less than half the support of the conservatives and puts it just one point ahead of the far-right Alternative for Germany (AfD).

The "Yes" means Merkel could be sworn in as Chancellor by mid-March.