But cast members at the exhibition said they were not aware of the movie.
"Oh, well, you've got confirmation before us. We have no idea if that's happening," said Sophie McShera, who played assistant cook Daisy Mason.
"But we would all love to be part of the film," she said.
Laura Carmichael, who played Lady Edith Crawley in the series, said she also wasn't aware of the plans for a film.
"Well, tell my agent, because we're still waiting to know. We're hoping that will happen soon."
Emmy-winning writer and series creator Julian Fellowes also said that he hoped a film would be made.
"I think we've got a film in us. I hope it happens," he said.
The show, which ended in 2015, portrays the lives of the English aristocratic Crawley family and their servants amid the backdrop of such historical events as the sinking of the Titanic and World War 1.
Downton Abbey has aired in at least 150 countries.
• This article was originally published in The Times.