The British actress, who plays Dr. Elizabeth Corday on the long-running medical drama, said she was recently told that after seven years on the show, her contract would not be renewed.
In an interview with Radio Times magazine, Kingston, 41, said the program was increasingly focused on young characters, "and apparently I, according to the producers and the writers, am part of the old fogeys who are no longer interesting."
"In that respect it's a shame. It's fine to have young med students, but you need to have figures of authority, people of different ages, races, shapes and sizes," she said.
Kingston said being let go "was a shock and obviously upsetting, but it's just your ego that's upset. Then I got over it."
Kingston, who starred as a warrior queen in the British drama "Boudica" (search) last year, said she plans to return to Britain for film and television work
"Scariness comes from not knowing whether I can do it any more, because I've been playing one role for so long," she said. "Admittedly, I've done little bits and pieces, but I'm very excited about getting back to the U.K."