US star Paul Giamatti is joining the cast of Downton Abbey for the Christmas special.

The American star, nominated for a best supporting actor award for his role in boxing drama Cinderella Man, will play Lady Grantham’s brother.

Shirley MacLaine returns as Lady Grantham’s mother Martha and James Fox joins the cast as aristocrat Lord Aysgarth.

The two-hour stand-alone episode of the award-winning ITV series, which will be shown at 8.30pm on Christmas Day, picks up the story six months on from the end of series four.

The plot revolves around Lady Rose MacClare’s (Lily James) entry into London high society where the family and servants take up residence at their home in the capital.

Much of the detail is being kept under wraps, but Downton bosses said Isobel Crawley ( Penelope Wilton) will find herself with an admirer, while Lord Grantham (Hugh Bonneville) has to protect the royal family and his own when a scandal threatens to engulf them.

Lancaster House in central London stood in for Buckingham Palace and filming was also done in West Wittering in Sussex and Basildon Park in Berkshire.

It goes up against EastEnders and Mrs Brown’s Boys on BBC1 in the Christmas ratings battle.

The period drama, which is returning to ITV for a fifth series next year, has been sold to more than 220 territories and watched by an estimated 120 million people around the world since its debut in 2010.