American actor Carrie Fisher, who played Princess Leia in the Star Wars films, is in a critical condition after a heart attack.
The 60-year-old went into cardiac arrest while on a flight from London to LA.
The LA Times reported passengers attempting to revive her with CPR and said she was rushed to hospital when the plane landed.
The celebrity news website TMZ.com and other news outlets also reported Fisher suffered a heart attack on the plane.
TMZ said she had the heart attack about 15 minutes before the plane landed at Los Angeles International Airport, and that an emergency medical technician who was on board had come to the first-class cabin to help.
United Airlines issued a statement saying that Flight 935 from London to Los Angeles was met on the ground by medical personnel after the crew reported that a passenger was "unresponsive," but the airline did not name the stricken passenger.
Twitter messages posted by other passengers aboard the plane reported that Fisher had fallen ill, with one saying she had stopped breathing.
Fisher had been on tour promoting her latest book, The Princess Diarist.