British Airways’ parent company, headed by chief executive Willie Walsh, announced earlier this week that it was buying the former Air Berlin subsidiary for €20 million, as well as providing cash of up to €16.5 million.

But now Fairplane, a group made up Niki passengers, has started legal action to move the airline’s insolvency proceedings from Germany to Austria in an attempt to recover €1 million in money owed by Niki to passengers.

Air Berlin liquidators said that this legal move, if successful, could “greatly endanger” the sale of Niki to IAG.

“If the complaint before the Charlottenburg court [in Germany] succeeds, the sale of Niki to IAG would be greatly endangered,” said a spokesperson for the liquidator.

IAG has not yet commented on this potential problem with the purchase of Niki.

Niki stopped flying in December 2017 after Lufthansa withdrew its bid to buy the airline as part of a wider purchase of Air Berlin assets.