The 53-year-old actor will play an American guest attending a wedding on the country estate, according to The Sun, which first reported the news.

"This is the biggest moment in Downton history and shows it's now the biggest drama in the world," a source told the newspaper. "All the plans have been shrouded in secrecy because George was determined for his role not to leak out."

Hugh is close friends with George - the pair starred in Nazi war drama The Monuments Men earlier this year - and it seems that he helped orchestrate the cameo for Downton during their time filming together.

"George and Hugh [Bonneville, who plays Lord Grantham] became really close during filming and it became very clear that George loved Downton," said the source.

"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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