Hollywood superstar George Clooney is to take a guest role in TV hit Downton Abbey - for a charity sketch.

The Oscar-winning heartthrob, soon to marry British fiancee Amal Alamuddin, will appear in a one-off special film to be screened in December.

The actor has previously worked with Downton star Hugh Bonneville - who plays Lord Grantham - on the film The Monument Men.

Clooney was spotted at Highclere Castle, the location used for filming of the series, earlier this year alongside lawyer Alamuddin.

His appearance will feature in a Downton Abbey sketch which will be part of the ITV charity extravaganza Text Santa to be screened later this year.

A programme insider confirmed his involvement in the project. But the other actors appearing alongside him and the storyline are being kept under wraps.

Clooney, 53, has landed Academy Awards for best supporting actor in Syriana and was producer of Argo which was named 2012's best film.