The handsome star is set to play an American wedding guest in a special show written to raise money for charity fundraiser Text Santa, and which will air at Christmas.
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George Clooney is set to appear in a Downton Abbey charity sketch
An ITV spokesperson confirmed that George would appear in a sketch, but would not act in a full Christmas episode. He has already filmed his scenes, and it's thought he will be seen on screen alongside Hugh Bonneville and Michelle Dockery.
George is reportedly a big fan of Downton Abbey, and is good friends with one of its stars, Hugh, who plays Lord Robert Crawley. The pair became close while working together on Monuments Men.
Earlier this year it was reported that George and his fiancée Amal Alamuddin had chosen Highclere Castle, the estate where Downton is filmed, as the venue for their wedding.
It's thought he will appear alongside friend Hugh Bonneville and Michelle Dockery
But, it seems, he was visiting the set for a different reason entirely.
"All the plans have been shrouded in secrecy because George was determined for his role not to leak out," a source told the Sun.
"All the other cast members and crew were 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."