Clooney 'has already filmed scenes with Downton Abbey cast'

George Clooney is reportedly set for a one-off a sketch alongside the cast of 'Downton Abbey' as part of this year's 'Text Santa' charity appeal on ITV.

A source told The Sun newspaper: "This is the biggest moment in 'Downton' history and shows it's now the biggest drama in the world.

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

The star has already filmed his scenes for the TV special, which will see his American character attend a wedding at the famous Yorkshire country estate, and the episode will air on the same night as the annual charity event, which aims to raise money for UK charities during the Christmas period, in December.

Clooney took on the role after striking up a friendship with Hugh Bonneville - who plays Robert Crawley in the period drama - when they both starred in the comedy 'The Monuments Men' earlier this year.

The source explained: "George and Hugh became really close during filming and it became very clear that George loved 'Downton.'

"He would joke around about the idea of getting a part so Hugh helped put the wheels in motion to make it happen."