EasyJet had proposed to acquire several of those assets, which included slots at Berlin Tegel airport, and will result in the British carrier operating 25 aircraft from Tegel, leasing some former Air Berlin aircraft and offering employment to Air Berlin flying crews.
Air Berlin had already warned that Lufthansa's was the only "viable" bid for Niki and that the carrier would have to file for bankruptcy if the European Commission failed to clear the acquisition before 21 December.
Niki has nearly 1,000 employees, and grounding the airline will result in tens of thousands of travelers being stranded, with hundreds of thousands of tickets becoming voided, Air Berlin said Tuesday. "Our decision enables easyJet to grow its presence at Berlin airports and start competing on new routes to the benefit of consumers", Vestager said in a statement.
The commissioner in charge of competition policy, Margrethe Vestager, said Tuesday that Easyjet's plans will not reduce competition.
Tough competition and falling ticket prices have led to the demise of Monarch and Air Berlin while Alitalia has filed for insolvency protection.
At least Air Berlin's insolvency administration is unlikely to face resistance from the European Union (EU) should they succeed in selling Niki to Thomas Cook at the last minute.
Niki said earlier that it had applied to open insolvency proceedings following Lufthansa's announcement that it had "informed Air Berlin and its insolvency managers that it will pursue the planned transaction without the purchase of Niki".
Separately, on 27 October 2017, Air Berlin announced the signing of an agreement with easyJet for certain other Air Berlin assets.
easyJet is an airline based in the United Kingdom that operates in the European short-haul aviation market and is focussed primarily in Western and Northern Europe. The German government was also relying on a Lufthansa purchase to help repay a 150 million-euro ($176 million) loan it had advanced to keep Air Berlin from being grounded in August.
However, EU antitrust officials were wary of granting Lufthansa too much market power and weren't willing to accept the remedy package the German airline had proposed.