Etihad Airways and Air Berlin have been blocked by German regulators from operating 34 codeshare routes this winter.
The codeshare routes affected by the ruling include Air Berlin flights from Berlin Tegel to 20 destinations in Europe and North America, and from Berlin Tegel and Stuttgart Echterdingen to Abu Dhabi.
All 34 codeshares have been cleared in the past. Etihad is currently under investigation in Germany over whether it "exercises too much control over" Air Berlin, reports Reuters.
Etihad owns a 29 per cent stake in Air Berlin.
Luftfahrt-Bundesamt (LBA), Germany's federal aviation office, declined the application because it said the codeshares contravene the country's Bilateral Air Services Agreement (BASA) with the UAE.
The agreement stipulates that UAE airlines may operate flights and codeshare services to four airports — Frankfurt International, Dusseldorf, Munich and Hamburg Fuhlsbüttel. They may then operate codeshare services from those four airports to Berlin, Stuttgart and Nuremberg.
Both airlines said they will challenge the ruling.
Etihad said in a statement: "These codeshares have been consistently approved by the LBA, with the German Ministry of Transport's full knowledge, support and agreement, for the last six seasons and have delivered greater choice to German business and leisure travellers while improving both Air Berlin and Etihad's ability to compete with the much larger Lufthansa Group in the German market.
"We are calling on the authorities to immediately reverse this unprecedented, unilateral and incorrect decision which, if implemented, will be detrimental German consumers, a German airline (Air Berlin is and will remain proudly German), German jobs and to Germany's connectivity and competitiveness."
Etihad.com
Airberlin.com