Air Berlin said last month it was in advanced talks to sell a majority shareholding in TopBonus and that it expected to conclude a deal by the end of this year. It declined to give details on the potential buyer at that time.

Asked to comment on Wednesday, Air Berlin and Etihad, which owns a 29.12 percent stake in the German carrier, only said they would disclose more details once negotiations were concluded.

The Austrian regulator said on its website that it was informed of the merger plans on December 4.

Air Berlin, which has not made an operating profit since 2007, plans to disclose details of a plan to rationalise the business by the end of the year.

The airline, Germany's second biggest after Lufthansa (LHAG.DE), had said last month it was confident the sale of TopBonus would help narrow the operating loss for this year.

Its shares were up 0.4 percent at 1.36 euros at 1204 GMT.

(Reporting By Marilyn Gerlach in Frankfurt and Stanley Carvalho in Abu Dhabi; Editing by Greg Mahlich)

• A Permanent Economic Emergency