Hollywood A-lister George Clooney is reportedly heading to Downton Abbey.

According to the Sun, the 53-year-old actor will take on the role of an American character in a sketch which will air at Christmas.

Clooney, who a huge fan of the period drama as well as good friends with Bonneville, who plays Lord Robert Crawley, is said to have already filmed the scenes where he plays a guest at a wedding in the country estate.

"All the plans have been shrouded in secrecy because George was determined for his role not to leak out," an insider told the newspaper.

"All the other cast members and crew were also sworn to secrecy. It's amazing it's taken four months for someone to find out. This is the biggest moment in Downton history and shows it's now the biggest drama in the world."

Although ITV is keeping tight-lipped about the mega coup, a spokesperson did confirm that the sketch in with Clooney will appear was written to raise money for a charity fundraiser, called Text Santa.

His forthcoming appearance will come as a surprise to some fans after Clooney previously denied rumours he was in talks with show bosses.

"I am too lowly for a part, I don't think they want me," he told the Sun in February. "I can't do the accent either. It's all very Dick Van Dyke."

Series five of Downton Abbey returns to ITV on 21 September.