United Continental has named its general counsel Brett Hart as acting chief executive, as well as confirming reports that CEO Oscar Munoz suffered a heart attack last week.
The airline said that Munoz was on medical leave following a heart attack on October 15, and that it was "too soon to know the course of treatment and timing of recovery."
Hart's appointment was effective immediately, United said in a statement released late Monday local time, adding that he will work closely with Henry Meyer, the non-executive chairman, and the executive team to run the company in Munoz' absence.
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Munoz' health problems come barely a month after he took on the job of improving the profitability and reputation of United, the No. 2 U.S. carrier by capacity.
United tapped Munoz to take over as CEO after it ousted Jeff Smisek in September amid a probe into improprieties at the Port Authority of New York and New Jersey. Smisek, who was chairman, president and chief executive, received about $4.9 million as a separation payment.
The company had until Monday declined to give details on the CEO's health, other than to say on Friday that he had been admitted to hospital.
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United has been criticized for not updating shareholders fully on its leadership situation.
"We need to know who is speaking on the behalf of the company and who is accountable for the decisions of this great airline," said Jeffrey Sonnenfeld, a corporate governance expert and a professor at the Yale School of Management, told Reuters.
While United might be waiting to provide comment out of respect for Munoz' privacy, the company also has an obligation to shareholders, Sonnenfeld said.
"Munoz is entitled to his privacy if he doesn't want to be CEO," he said.
—CNBC's Everett Rosenfeld contributed to this report.