The European Union's executive, which enforces environmental law, said on Friday it had exchanged letters with the German government because the airport's flight paths were different to those endorsed in a EU environmental impact assessment.

Joe Hennon, the Commission's environment spokesman, said the new routes would take planes over lakes and other habitat used by protected birds.

"The final flight routes are not the same as those initially announced. We now have to consider our position," he said.

"One of the options would be to start a formal infringement procedure but we are not there yet." A decision will be made in the coming weeks, he said.

The threat of EU legal action will further compound the public relations fallout surrounding Willy Brandt International Airport, where repeated delays to its opening have already tarnished Germany's reputation for efficiency.

Although such an infringement process could end up in the European Court of Justice, most such cases are settled in advance, with countries altering the projects to comply with EU environmental rules.

It is unlikely the Commission would order another postponement to the airport's opening, although its complaint may mean that flight paths may have to change.

Earlier this week, Berlin Mayor Klaus Wowereit stepped down as head of a board overseeing the project after delays, caused by red tape and, latterly, problems with the fire-safety system, turned the airport and its city into a butt of nationwide jokes.

Already 20 years in the planning when construction began in 2006, the airport was then set to open in 2011. That was pushed back to June 2012, then to March 2013, then to October 2013 and now to some point in 2014 or even later.

The new airport is named after Willy Brandt, the late former West German chancellor and Nobel Peace Prize laureate. (Reporting By John O'Donnell; Editing by Pravin Char)

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