The Hollywood star has already filmed his scenes for the upcoming episode, which will air in December as part of ITV's Text Santa event, according to The Sun.
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George Clooney
George, who accepted the role after becoming good friends with Hugh Bonneville (Lord Robert Crawley) on the set of their film 'The Monuments Men', will play an American who visits the country estate to attend a wedding.
"All the plans have been shrouded in secrecy because George was determined for his role not to leak out.
"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."
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The cast of 'Downton Abbey'
George has made no secret of his love of the show in the past but always ruled out making an appearance because he believed he would struggle with the accent.
"We all love that show, it's huge. I am too lowly for a part, I don't think they want me," he said earlier this year. "I can't do the accent either. It's all very Dick Van Dyke."
The 'Ocean's Eleven' star was spotted at Highclere Castle, where 'Downton' is filmed, earlier this year but we were led to believe he was considering the venue as a potential wedding location.
George Clooney arriving for the UK Premiere of The Monuments Men, at the Odeon Leicester Square, London. (credit:PA)
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PARIS, FRANCE - FEBRUARY 12: George Clooney attends 'Monuments Men' Paris premiere at Cinema UGC Normandie on February 12, 2014 in Paris, France. (Photo by Julien M. Hekimian/Getty Images) (credit:Getty Images)
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MILAN, ITALY - FEBRUARY 10: (EDITORS NOTE: Image has been converted to black and white) George Clooney attends 'The Monuments Men' Milan Premiere on February 10, 2014 in Milan, Italy. (Photo by Vittorio Zunino Celotto/Getty Images) (credit:Getty Images)
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BERLIN, GERMANY - FEBRUARY 08: George Clooney speaks on stage at 'The Monuments Men' premiere during 64th Berlinale International Film Festival at Berlinale Palast on February 8, 2014 in Berlin, Germany. (Photo by Ian Gavan/Getty Images) (credit:Getty Images)
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BERLIN, GERMANY - FEBRUARY 08: George Clooney attends 'The Monuments Men' premiere during 64th Berlinale International Film Festival at Berlinale Palast on February 8, 2014 in Berlin, Germany. (Photo by Andreas Rentz/Getty Images) (credit:Getty Images)
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US actor George Clooney poses during a photocall for the film The Monuments Men at the 64th Berlinale Film Festival in Berlin, on February 8, 2014. The 64th Berlinale, the first major European film festival of the year, starts with 24 international productions screening in the main showcase. AFP PHOTO / JOHANNES EISELE (Photo credit should read JOHANNES EISELE/AFP/Getty Images) (credit:Getty Images)
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BEVERLY HILLS, CA - NOVEMBER 09: Actor/filmmaker George Clooney accepts the Stanley Kubrick Britannia Award for Excellence in Film onstage during the 2013 BAFTA LA Jaguar Britannia Awards presented by BBC America at The Beverly Hilton Hotel on November 9, 2013 in Beverly Hills, California. (Photo by Kevin Winter/Getty Images) (credit:Getty Images)
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BEVERLY HILLS, CA - NOVEMBER 09: Actor George Clooney attends the 2013 BAFTA LA Jaguar Britannia Awards presented by BBC America at The Beverly Hilton Hotel on November 9, 2013 in Beverly Hills, California. (Photo by Jason Merritt/Getty Images) (credit:Getty Images)
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HOLLYWOOD, CA - NOVEMBER 08: Producer George Clooney attends the premiere of The Weinstein Company's 'August: Osage County' during AFI FEST 2013 presented by Audi at TCL Chinese Theatre on November 8, 2013 in Hollywood, California. (Photo by Kevork Djansezian/Getty Images for AFI) (credit:Getty Images)
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NEW YORK, NY - OCTOBER 02: Actor George Clooney attends an official screening of 'Gravity' for Academy members hosted by The Academy of Motion Picture Arts and Sciences on October 2, 2013 in New York City. (Photo by Brad Barket/Getty Images for Academy of Motion Picture Arts and Sciences) (credit:Getty Images)
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VENICE, ITALY - AUGUST 28: Actor George Clooney attends the Opening Ceremony And 'Gravity' Premiere during the 70th Venice International Film Festival at the Palazzo del Cinema on August 28, 2013 in Venice, Italy. (Photo by Ian Gavan/Getty Images) (credit:Getty Images)