Ahead of the Christian Social Union convention in southern Germany, Chairman of the CDU/CSU group in the European Parliament, Werner Langen, has made it clear that Merkel's party does not support its sister party's demands.

"The German government has to be capable of acting and negotiating in Brussels," Langen told the Muenchner Merkur newspaper, adding that an imperative mandate for Berlin would turn Germany into the lame duck of the EU.

Earlier this month CSU chairman Horst Seehofer said he wanted both houses of the German parliament to have a greater say in EU matters affecting Germany.

But CDU European Member of Parliament Elmar Brok said the demands are unrealistic.

"We have to secure the EU's ability to act," Brok told ARD television on Tuesday. He said it would be a poor deal if all the other member states did all the negotiating while Germany was not involved.

Constitutional Court ruling

The current discord between the CDU and CSU started when the Constitutional Court in Karlsruhe ruled that Germany would have to make some slight amendments to domestic laws before it could ratify the Lisbon Treaty.

Chancellor Merkel had been hoping to rush through the amendments, which relate to how much say Berlin has in the formulation of EU decisions, with time to spare ahead of German elections in September.

But unless the CSU backs away from its hardliner, that target could be hard to reach.

Merkel is due to join the Christian Socialist convention on Tuesday, where the focus is likely to be on the parties' diverging views of Germany's role in Brussels.

Earlier, Merkel told her executive committee that the CDU has been Germany's Europe party since Adenauer and Kohl, and that that was a legacy which had to be continued.

FDP warning

Liberal European MP, Silvana Koch-Mehrin has also rejected CSU calls. She told the Muenchner Merkur that although it was quite right to want to strengthen democratic and parliamentary rights, there were limits.

"Anyone who, like the CSU, is prepared to sacrifice the Lisbon Treaty as part of an election campaign tactic cannot rely on the support of the Free Democrat Party."

Germany, Poland, Ireland and the Czech Republic are the only countries that have not yet ratified the treaty.

tkw/AP/dpa