Leaders of Germany’s centre-left Social Democrats (SPD) have suffered a blow after the party’s youth section voted “no” to plans for a “grand coalition” with Chancellor Angela Merkel’s conservatives.

The country’s future government depends on the unprecedented say of the SPD grassroots. Many youth members fear the party would simply be gobbled up in a coalition by the more right wing Christian Democrats (CDU).

“I want to send a warning to my party. Rejecting the coalition agreement Johanna will not mean more justice in Germany, rejecting it will mean less social justice for millions of people,” SPD leader Sigmar Gabriel, told the “Jusos” – the young Social Democrats – at their Congress in Nuremberg.

The coalition deal hammered out after weeks of wrangling grants SPD wishes for a minimum wage and a lower retirement age, but rules out tax rises.

The agreement came after in the wake of September’s general election which Merkel’s CDU won handsomely but without gaining an overall majority.

The ballot of the whole SPD membership is open until Thursday 12 December; the result is expected at the weekend.