"Because of our experience with Bulgaria and Romania, we must take the European Commission's latest progress report seriously. Croatia obviously is not yet ready for entry," Lammert, a member of the Christian Democratic Union (CDU), said in an interview with the Sunday edition of Die Welt newspaper.

Lammert said that the countries of the former Yugoslavia had a prospect of EU membership, but that they themselves should lay the ground for entry. He said that only concrete results in meeting the EU membership requirements should be taken into account, while "good intentions and promises should not become a substitute for proof of reform."

The Speaker of the Bundestag said that the EU was not capable of further enlargement "in the foreseeable future", but needed consolidation and stabilisation.

His statement received severe criticism from the German Social Democratic Party (SPD).

"Whoever calls Croatia's entry into question believes that the European Union is incapable of making peace in Europe in the future. This is the most mistaken understanding of the award of the Nobel Prize for Peace to the European Union," the Minister for European Affairs of the federal state of Baden-Wuerttemberg, Hans-Peter Friedrich of the SPD, told Spiegel Online, the online version of Der Spiegel news magazine.

"Neither are previous mistakes being repeated in the case of Croatia, nor should Croatia be punished for the mistakes made by others. The Balkans is still of crucial importance to peace in Europe," Friedrich said.

Before Lammert, another Christian Democrat also spoke against Croatia's accession to the EU. "At the moment Croatia is not ready for EU entry," the chairman of the Bundestag European Affairs Committee, Gunther Krichbaum, told Saarbrueckener Zeitung daily on Friday, adding that Zagreb could not count 100 per cent on the scheduled date of accession, July 1 next year.

CDU member of the Bundestag Michael Stuebgen said that ratification of Croatia's EU accession treaty, which is planned for spring, should be contingent on a favourable final European Commission progress report, which is due in spring.

"If the next European Commission report is also so critical, it could be problematic," Stuebgen said. Michael Roth of the SPD, who is in charge of European affairs, made a similar statement.

The Green Party was explicitly against threatening Croatia with postponement of accession. "The Bundestag should exert pressure, but threats of a veto are dangerous," Green Party spokesman for European affairs Manuel Sarrazin said, noting that Croatia "must do its homework." He said he would recommend to his party colleagues to vote in favour of ratification of Croatia's EU accession treaty.

CDU/CSU have expressed their opposition to the EU enlargement process before, but until now they always mentioned Croatia as an exemption.