Shirley MacLaine, who is joining the Downton Abbey cast. Photograph: Paul Faith/PA

Her withering asides and perfectly-timed eye-rolls have long been the highlight of Downton Abbey, but the dowager countess, as played by Maggie Smith, might be about to find herself with some competition. For Shirley MacLaine is joining the show as Lady Grantham's mother.

MacLaine, the Oscar-winning star of Terms of Endearment, The Apartment and In Her Shoes, will play Martha Levinson – mother of American heiress Cora, whose substantial fortune was initially responsible for her marriage to Lord Grantham – in the third series of Downton, which begins filming next month and will be set in 1920 and 1921.

Gareth Neame, executive producer of the show, said: "My late grandfather directed Shirley MacLaine in Gambit in 1966 so it is a delight for me that she will be joining us on Downton Abbey.

"Julian Fellowes has written another brilliant character in Martha Levinson, who will be a wonderful combatant for Maggie Smith's dowager countess and we are excited at the prospect of Shirley MacLaine playing her."

MacLaine, who has made more than 60 films, made her debut in Alfred Hitchcock's 1955 movie The Trouble with Harry; her performance winning her the first of seven Golden Globes that she has amassed over a career that has included movies such as Some Came Running (alongside Dean Martin and Frank Sinatra), Irma la Douce (with Jack Lemmon) and Steel Magnolias.

Downton Abbey, a Sunday-night phenomenon for ITV in Britain, has proved a hit on both sides of the Atlantic – bringing excellent ratings in recent weeks for the American public service broadcaster PBS, as American audiences embrace Fellowes' tale of life above and below stairs in a grand English house.

Earlier this month the show picked up a Golden Globe for best television mini-series to go with the four Emmy awards it won in 2011 – with rumours of a Downton Abbey film since circulating. Fellowes wrote the screenplay for the 2001 movie Gosford Park.

Laura Mackie, director of drama at ITV, said: "It is so exciting to have an actress of Shirley MacLaine's stature joining our brilliant Downton Abbey cast for series three and a tribute to the show's success on both sides of the Atlantic."