I hope the agreement will be reached, otherwise the consequences could be dramatic, Lammert said at a conference of parliament speakers from EU and accession candidate countries.
He said that unless a constitutional agreement was adopted, neither Croatia nor Ukraine nor Turkey could become EU members.
The German Bundestag will not approve new enlargement rounds either if it transpired that the EU could not function, said Lammert.
EU countries are currently divided over how much to alter the initial draft of the constitutional contract, how to call it and what would be the best way to ratify it.
Lammert is expected to visit Croatia next week.