London (CNN Business) The chief executive of one of Europe's largest low-cost carriers stepped down Thursday from the company he founded and still partially owns.

EasyJet ESYJY Norwegian Air's Bjoern Kjos said that he would continue working as an adviser to the airline's chairman, focused on industry alliances and building on a recent agreement with rival

Chief Financial Officer Geir Karlsen will act as interim CEO while the company hires a permanent replacement.

Norwegian Air shook up global aviation by offering discounted flights within Europe and to the United States, Asia, and Latin America. Kjos owns part of a 17% stake in the carrier through a holding company under his control.

The company's future has been called into question after a rapid expansion left it in debt. In January, it sought to raise 3 billion Norwegian kroner ($352 million) to shore up finances.

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