Veteran British actress Dame Maggie Smith is convinced she will never work in theatre again due to her advancing years.

The 78 year old is an accomplished stage performer, having started her career in the theatre and tackled many Shakespearean roles over the years, notably playing Desdemona in Othello opposite Sir Laurence Olivier in London in 1964.

Smith hasn't been seen on stage since her 2007 turn in The Lady From Dubuque at London's Theatre Royal Haymarket, and the actress now admits she is too old to go back.

She tells U.S. Tv show 60 Minutes, "I feel I won't work in the theatre again. I'm sad about that. But I won't retire."

Smith insists she will continue working on film and Tv projects such as period drama Downton Abbey, in which she plays ageing aristocrat, Violet, Dowager Countess of Grantham, and suggests the only way to make her leave the show will be to "kill off" her character.

She adds, "I'll keep going with Violet and any other old biddy (role) that comes along... Logically, Violet must be about 100-and-something now, so I don't know how long she'll go on. There will be bathchairs and things they can push me around on. I think they'll have to (kill her off)."