Downton Abbey faces another American invasion as Sideways star Paul Giamatti to play Countess Cora's brother

Paul Giamatti has been cast in series four of Downton Abbey as uncle of Mary and Edith Crawley (AP)

Sideways star Paul Giamatti is set to join the cast of Downton Abbey in the fourth series from creator Julian Fellowes.

The American actor will play Harold Levinson, the maverick, playboy uncle of Mary and Edith Crawley, and brother of Countess Cora, in the next series of the ITV period drama which starts this Autumn.

He will also make an appearance in this year's Christmas special alongside returning American family-member Shirely MacLaine, who plays his mother Martha.

Giamatti, who earned an Emmy and a Golden Globe for his role in HBO’s John Adams, will join a number of new faces who have signed for the fourth series, including Tom Cullen, Nigel Harman, Julian Ovenden, Joanna David and Dame Kiri Te Kanawa.

Gareth Neame, Downton Abbey executive producer, described Harold as a “free-spirited” character. He promised that both Giamatti and MacLaine will “upset the Granthams' apple cart”.

The casting of another American star is a tactical move from the show’s producers as the drama continues its successful run in the US on PBS.

It has become the highest-rated drama in the channel’s history, attracting 8 million viewers by the end of series three, and winning six Emmy awards.