A secondary insolvency filing in Austria, which Niki will submit by the end of this week, will safeguard the sale, Lucas Floether said in a statement on Tuesday.

Niki filed for insolvency in Berlin last month after Germany's Lufthansa scrapped plans to buy the Austrian arm of Air Berlin, grounding its fleet and stranding thousands of passengers.

After hurried talks to find a new owner for Niki before it lost its valuable runway slots, IAG agreed to buy the business and make it part of low-cost unit Vueling.

But Fairplane, a group representing airline passengers, threw a spanner in the works, filing legal cases last week to have Niki's insolvency proceedings shifted to Austria.

A regional court in Berlin backed Fairplane's position this week and said it would reverse the opening of insolvency proceedings in Berlin. Niki has filed an appeal against the ruling with Germany's supreme court, which said on Tuesday it would handle the case "swiftly".

Meanwhile Niki is burning through up to 16.5 million euros ($19.7 million) of interim financing it has been given by IAG's Vueling, which administrator Floether said would last for only a few weeks.

IAG declined to comment on Tuesday.