George Clooney, the Oscar-winning American actor, has signed on to star in a special episode of ITV's 'Downton Abbey'. Clooney will play a visiting guest from the U.S who attends a wedding at the country estate.

George Clooney [L] and Hugh Bonneville [R] will appear together in 'Downton Abbey'

The show will be broadcast in the UK at Christmas and was written as part of the charity fundraiser Text Santa, for ITV. A spokesman for the channel confirmed that Clooney had already filmed his scenes for the sketch but said he would not appear in a full episode. The spokesperson refused to give away any further details but it is believed that the actor will appear alongside Hugh Bonneville and Michelle Dockery.

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Clooney is known to be good friends with Bonneville - the pair starred together in The Monuments Men - and is believed to be a fan of the multi-award winning show. Tabloid reports suggest the Gravity star considered choosing the Highclere Castle, where Downton is filmed, as the venue for their upcoming wedding.

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Sources told the Sun: "All the plans have been shrouded in secrecy because George was determined for his role not to leak out.

"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."

Last year, Text Santa featured a special Coronation Street sketch with Cheryl Cole working behind the bar at the Rovers' Return.

Watch George Clooney and Hugh Bonneville in 'The Monuments Men' trailer:

