Harry Potter's Maggie Smith was the big wig in "Downton Abbey," but now she'll have some competition from an unlikely source -- Shirley MacLaine.

The 77-year-old actress will play Martha Levinson, the mother of Lady Grantham, played by Elizabeth McGovern. It will likely be a battle of the in-laws in the Downton house, with Smith already playing Violet, the mother of Hugh Bonneville's Earl of Grantham (and McGovern's on-air husband).

The series, which has won massive critical acclaim, has already earned a third season, which begins shooting in the spring.

"Shirley MacLaine is a great actress and she's as American as the day is one," said Rebecca Eaton, executive producer of "Masterpiece," which airs the show in the United States. "I can't wait to see her go toe-to-toe with Maggie Smith's Lady Violet."

MacLaine is a celebrated actress whose career stretches back to 1955 when she appeared on "Shower of Stars" and "The Trouble with Harry." She was nominated for five Oscars between 1961 and 1978, and won on her sixth try in 1984 for "Terms of Endearment."

MacLaine has also been nominated for six Emmy awards, winning once in 1976 for "Gypsy In My Soul." She was last nominated for an Emmy in 2009 for "Coco Chanel."

She may be as American as you can get, but MacLaine is a darling of foreign audiences, especially in Europe. MacLaine has been nominated for 19 Golden Globes awards, winning five of them. That doesn't even include two special awards she received -- one in 1959 for Most Versatile Actress, and another in 1998, the coveted Cecil B. DeMille Award, given to those who have made an outstanding contribution to the entertainment field.

MacLaine is joining a show that has been recognized for awards itself. It was nominated for 11 Emmy awards last year, winning six of them, mostly in the miniseries category. It was also nominated for four Golden Globe awards, winning one. And that was all just for Season 1.

"Downton Abbey" is making its American run right now on PBS' "Masterpiece Classics."