"A country mature for accession, such as Croatia, should not become a hostage to the Treaty of Lisbon and its accession should not be tied to the entry into force of that treaty. We need both: Croatia and the reform treaty," Posselt said in a statement on Tuesday.
His statement came as a response to statements made by officials of the Social Democratic Party (SPD), a coalition partner in the government of Chancellor Angela Merkel, that Croatia's EU entry should be put on hold if the Lisbon Treaty collapsed.
The Tagesspiegel daily on Tuesday quoted the SPD expert on European affairs, Axel Schaefer, as saying that Croatia should not be admitted to the European bloc in case of failure of the Lisbon Treaty.
The destiny of the Treaty of Lisbon, which paves the way for structural changes in the EU as well as for its further expansion, is uncertain following the resignation of the government of the Czech Republic, which has not yet ratified the treaty.
The CSU and Angela Merkel's CDU are pushing for suspension of the EU enlargement process after Croatia joins the 27-member bloc.
Posselt said that, considering earlier promises, it would be unjust to leave Croatia, as a pro-EU country, out of the enlargement process, rather than punish those blocking the process.
Posselt recommended that the German Foreign Minister, Frank-Walter Steinmeier of the SPD, should advocate the prospects of EU membership for Macedonia and other southeastern European countries and should halt Turkey on its path towards the EU.