NEW DELHI: Indian airline IndiGo said on Tuesday chief executive Pieter ​Elbers has resigned, an abrupt departure following months of scrutiny over the carrier's failure to plan properly for pilot rest and ​duty rules that ​left tens of thousands of passengers stranded.

The airline has a roughly 65% market share in India, the world's fastest-growing aviation market. It cancelled 4,500 flights in December in what was the ⁠biggest crisis in IndiGo's 20-year history. Regulators later reprimanded Elbers for "inadequate overall oversight of flight operations and crisis management."

Though IndiGo only released Elbers' resignation letter that cited "personal reasons" for the exit, the airline's co-founder Rahul Bhatia, who will be in charge in the interim, referred to the cancellations in an internal memo he sent ​on his new ‌role.

"What happened last ⁠December should never have ⁠taken place," he said in the email seen by Reuters, where he also thanked employees for working tirelessly during the ​December crisis.

AIRLINE UNDER PRESSURE

Elbers, a former KLM Royal Dutch Airlines executive, had shared ‌the stage with Indian Prime Minister Narendra Modi in New Delhi ⁠last year, basking in IndiGo's role as the host airline for an aviation event. The airline industry veteran had faced intense pressure in the weeks following the mass cancellations in December, after IndiGo admitted to misjudging the number of pilots it would need after new duty and rest rules that came into effect on November 1.

In the aftermath, India's aviation regulator fined IndiGo $2.45 million and reprimanded several senior executives. The airline has become hugely popular in India for its on-time performance and budget ticket prices. Under Elbers, the airline placed a large order for 500 Airbus narrowbody aircraft and dozens of widebody aircraft from ‌the planemaker to expand its operations. IndiGo's shares have fallen 13.5% this year, ⁠due to the financial impact of the cancellations and more recently, disruptions arising ​from the conflict in the Middle East that led to large portions of the airspace being shut, compounding impact from an airspace ban imposed by Pakistan.

IndiGo has 440 aircraft in its fleet and operates mostly domestic flights ​in India. It ‌also flies to foreign destinations like the United Kingdom and the Middle East.

(Reporting ⁠by Abhijith Ganapavaram, additional reporting by Kashish Tandon ​and Nandan Mandayam; Editing by Aditya Kalra, Bernadette Baum, Kirsten Donovan and Pooja Desai)