The show, which will be broadcast at Christmas, was written to raise money for a charity fundraiser, called Text Santa for ITV. An ITV spokesperson said Clooney would appear in a sketch, but would not act in a full Christmas episode.
The Hollywood star has already filmed the scenes but a spokesperson for the period drama refused to give away any further details. However, it was reported that Clooney was set to appear alongside Hugh Bonneville and Michelle Dockery.
Clooney is reportedly a huge fan of Downton Abbey and is good friends with Bonneville, who plays Lord Robert Crawley. The pair became close after working on the war film, Monuments Men, together.
Earlier this year, it was also claimed that Clooney and his fiancée, Amal Alamuddin, considered choosing Highclere Castle, the Hampshire estate where Downton Abbey is filmed, as the venue for their upcoming wedding.
However, it has been suggested that this was an elaborate cover story – invented after he was spotted at the castle – in order to keep his Downton appearance a secret.
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."
The Text Santa fundraising appeal last year saw Cheryl Cole star in a special episode of Coronation Street. The singer was shown serving drinks in the Rovers Return, alongside Michelle Keegan, who plays Tina McIntyre.
ITV said that all money raised from the show will go to charity.
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