Stars including Hugh Bonneville and Elizabeth McGovern were spotted in 1920s costume for what appeared to be a party scene at the historic estate near Leeds in October.
Bonneville, who plays Downton's patriarch Lord Grantham, was snapped in black tie while his on-screen wife Lady Grantham, played by McGovern, was attired in period evening dress and a tiara.
Laura Carmichael, who plays their daughter Lady Edith, was also on the set, as was Penelope Wilton, who appears as family friend Isobel Crawley. Some of the actresses were wearing Ugg boots and puffa jackets to keep warm between takes.
As Highclere Castle in Hampshire is used as the location for the fictional Downton Abbey, is it likely Harewood is standing in for another country house, possibly owned by friends of the Granthams, in the upcoming movie, which is set in the 1920s and carries on from where the last series of the ITV drama ended.
Vintage cars were also spotted on the set. The film is due for release in autumn 2019.
It's not the first time Harewood has starred in a costume drama - it stood in for Buckingham Palace in the ITV series Victoria, about the life of Queen Victoria.
Downton Abbey is a fictional North Yorkshire estate and towns such as Easingwold, Thirsk and Ripon are referenced throughout the six series of the show.