The declaration suggests the grand coalition could come crashing down after just two years if the agreement isn’t working.
But Martin Schulz has continued to remain upbeat, saying: “We can not leave the impression that we are already at the end of the road.
“We have now had the soundings and have found it worthwhile to continue to negotiate with each other.
“No one should now leave the impression that it’s all over.”
German chancellor Angela Merkel finally reached an agreement with Mr Schulz’s SPD group this month in an attempt to claw back power after failing to form a government in Germany’s general election.
But SPD members have raised concerns a grand coalition will force them to lose their voice.
Additional reporting by Monika Pallenberg