BERLIN, March 2 (Xinhua) -- The supervisory board of the troubled Berlin Brandenburg international airport (BER) has defended a request by its operating firm for more public funds on Friday.

"From our perspective the concept is convincing", board chairman Rainer Bretschneider told press.

Bretschneider was hereby responding to a recent application by BER director Engelbert Luetke Daldrup for a further 100 million euros (123.3 million U.S. dollars) of government funding to begin construction of an additional terminal in 2020. Luetke Daldrup hopes to secure an outstanding amount of 400 million euros for the extra terminal on private capital markets.

So far, taxpayers have already contributed 6.5 billion euros to the giant infrastructure project which was first scheduled to open back in 2012. When BER was first announced, its total cost for what will become Germany's third largest airport were estimated at a mere 2.2 billion euros.

Following a series of problems, ranging from technical issues to the unexpected insolvency of BER's largest prospective customer Air Berlin, however, the date, at which BER will become operational, has been postponed to 2020.

The significant resulting delay means that the international airport will not have sufficient capacity to cope with an increase air traffic in the German capital since 2012, requiring either a costly expansion of the project or the prolonged operation of several of Berlin's older airports such as Tegel.

Luettke Daldrup initially anticipated additional costs of 770 million euros for the expansion, but then proposed resorted to a private-public leasing model which would lower the initial price tag. Critics have pointed out that the variant now preferred was likely to result in higher overall operating costs for BER in the long-run.