Airline Niki's administrator presses ahead with sale to IAG

* German court ruling puts question mark over sale

* IAG's Vueling has said it is still interested in Niki

* Filing in Austria to safeguard deal - administrator

* Interim financing to last only a few weeks - administrator

* Niki appeals against court ruling to supreme court (Adds comments from airline founder and previous bidder Niki Lauda)

By Maria Sheahan

FRANKFURT, Jan 9 (Reuters) - The administrator of Niki said he would press ahead with an agreed sale of the insolvent Austrian airline to British Airways owner IAG after a German court ruling fanned concern that the deal could unravel.

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 (Xetra: LHAB.DE - news) scrapped plans to buy the Austrian arm of Air Berlin (LSE: 0GPE.L - news) , 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".

Niki's founder, former Formula One world champion Niki Lauda, said on Oe24 TV he stood ready, if given the chance, to offer a fresh bid for the airline together with Thomas Cook (Frankfurt: A0MR3W - news) and Condor, after missing out to IAG last month.

"I continue to be interested but the question does not currently arise... The legal situation is not clear," Lauda said, adding he had previously offered around 36 million euros.

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.

($1 = 0.8384 euros) (Additional reporting by Ursula Knapp, Shadia Nasralla and Alistair Smout; Editing by Mark Potter, Adrian Croft and Jane Merriman)