Abu Dhabi-based airline Etihad has been denied approval to add its code to some flights operated by partner Air Berlin to Germany, meaning the German airline could lose out on passengers that it needs to fight its way back to profit.

Air Berlin said on Friday that it had been informed by Etihad, which applied for permission to add its number to Air Berlin flights, that 34 codeshare flights for the winter schedule had not been approved, affecting 46,000 bookings.

Etihad and Air Berlin share codes on flights to around 60 destinations. By putting its number on the German airline's flights, Etihad can sell those connections to its customers and thus help loss-making Air Berlin to fill its planes.

The German federal aviation authority (LBA) checks codeshare deals every time airlines submit winter and summer schedules for flights to and from Germany. The two airlines have shared flight codes since 2012 and the LBA has approved all six previous flight plans.

Air Berlin said passengers on the flights affected would still fly but would receive new tickets without an Etihad flight number. Flights for which codeshares have not been approved include Berlin and Stuttgart to Abu Dhabi, plus flights from Berlin tonorthern Europe, a spokesman for Air Berlin said.

Air Berlin, in which Etihad owns a 29 per cent stake, made its first annual net profit in five years in 2012, but only after Etihad bought a majority stake in its frequent flyer programme.

Air Berlin has previously said it expects code shares to bring it and Etihad additional revenue of €200 million ($253.4 million) in 2014, with over half going to Air Berlin.

"As the financial impact will be borne mainly by Air Berlin, the airline will take all legal steps within its power to fight this decision," Air Berlin Chief Executive Wolfgang Prock-Schauer said in a statement.

The LBA confirmed the decision but declined to comment further.

Etihad is already under investigation by German authorities as to whether it exercises too much control over Air Berlin. For Air Berlin to maintain its European operating licence, it must be majority controlled by European investors. - Reuters