Originally set to open on June 3, 2012, failure to meet fire safety regulations first forced authorities to say it would take a few more months, then to set a new opening date in March 2013. But even this date has been in doubt because of complications in routing smoke from the building through subterranean channels.

Airport authorities said on Wednesday the smoke extraction system had succeeded in channelling fumes from the building in a simulated power outage a day earlier, with no smoke leakage between different floors according to visual examination.

The long-awaited 2.5 billion euro airport will service Berlin and replace the two airports from the Cold War era. It should attract 27 million passengers a year, making it the third busiest airport in Germany after Frankfurt and Munich.

Airport spokesman Ralf Kunkel called it "the most important project for the future of the German capital".

The cost of keeping the city's soon-to-be-defunct airports open longer has been put at about 15 million euros per month. Airlines Lufthansa and Air Berlin have said they will seek compensation for losses incurred by the delay.

Airport authorities are still evaluating measurements and video recordings from the safety tests and have not yet released a full report. Berlin's business community remained sceptical.

"Just because we know that yesterday the fire alarm equipment didn't implode and managed to get the smoke out, it is not necessarily cause for optimism," said Joerg Nolte, spokesman for the Chamber of Commerce and Industry in Berlin.

Testing on the fire system will continue until December, but Tuesday's results are likely to have an important impact on the board of directors' final decision on the opening date of the airport, which will be determined on August 16. (Reporting by Samuel Frizell; Editing by Stephen Brown and Mark Heinrich)

• A Permanent Economic Emergency