WASHINGTON (Reuters) - Greg Ulmer, head of Lockheed Martin Corp's LMT.N F-35 fighter program, will temporarily lead the company's aeronautics division while its current chief, Michele Evans, undergoes treatment for a "significant" medical issue, Evans told employees on Monday.

FILE PHOTO: U.S. President Donald Trump greets Lockheed Martin CEO Marillyn Hewson in front of a Lockheed Martin F-35 stealth fighter on the driveway abutting the South Lawnat the White House in Washington, U.S., July 23, 2018. REUTERS/Carlos Barria/File Photo

Evans, 54, who took over as executive vice president of Lockheed’s largest division in October 2018, is widely seen as a leading candidate to succeed Marillyn Hewson, 65, as chief executive of Lockheed, the No. 1 U.S. arms maker.

The surprise announcement about Evans’ illness came at the start of the high-profile annual Air Force Association conference in Washington, and could raise questions about Lockheed’s succession planning.

Evans was diagnosed with cancer several months ago but has continued to work while undergoing treatment, according to three sources familiar with the situation.

Evans gave no details, but told employees about what she called “a significant medical issue” in a letter on Monday.

“While it’s certainly been an unexpected shock for me and my family, the prognosis is excellent and the condition is curable,” she wrote in the memo, a copy of which was viewed by Reuters.

She said she would remain engaged as much as possible, but Ulmer would serve as her delegate and acting executive vice president of the aeronautics division through late November, while she focused on her recovery.

Lockheed spokesman Joe LaMarca said the medical issue was discovered during Evans’ annual executive physical and follow-up with medical personnel.

“Her prognosis is excellent, the condition is treatable, and a full recovery is expected,” he said.

“Evans has been viewed as the leading internal candidate to succeed Marillyn Hewson, so people inside and outside the company will be following this story closely,” said Loren Thompson, a defense consultant with close ties to Lockheed.

Evans worked at multiple positions across Lockheed throughout her 32 years in the weapons industry, and plays a leading role in the company’s diversity initiatives.

Industry insiders and analysts say she is among the top candidates to succeed Hewson, given her age, strategic vision and broad range of experience across the company.

Lockheed last year changed its mandatory retirement age rules to exempt the CEO, allowing Hewson to stay on past this year. It has not named a chief operating officer to start the process of choosing a successor to Hewson, who last year topped Fortune magazine’s U.S. list of most powerful women.

Aeronautics, the largest of the company’s four divisions, reported a 15% increase in revenues to $11.13 billion in the first half of 2019, with operating profit up 13% to $1.18 billion in the same period.

With about 25,000 employees, Lockheed’s aeronautics division builds the F-35 stealth fighter, the C-130J transport plane, and a variety of other military aircraft. It also includes the company’s famed Skunk Works advanced development unit.