Smith stars as Violet Crawley, Dowager Countess of Grantham in the hugely popular franchise, but her involvement in the new project was uncertain following a moving scene in the last film where she told her granddaughter that she was sick and "may not have long to live."
Despite sources claiming producers are worried it would be impractical for Smith, who turns 86 in December, to return amid the pandemic, according to the MailOnline she's agreed to appear in the flick.
She'll be joined by her screen family, including Hugh Bonneville's Lord Grantham and Elizabeth McGovern as his wife Cora. Her granddaughter Lady Mary, played by Michelle Dockery, will also return, along with Matthew Goode as her husband Henry.
Meanwhile, Jim Carter's Mr Carson, Phyllis Logan's Mrs Carson nee Hughes, Brendan Coyle's Mr Bates and his wife Anna, played by Joanne Froggatt, will also be back, alongside Imelda Staunton's Maud, Lady Bagshaw and Tuppence Middleton as Lucy Smith, who joined the cast for the first movie.
The film will shoot from March until May next year under strict Covid-19 safety protocols. Creator Julian Fellowes has been "polishing" the screenplay during the lockdown.
The first film based on the British hit drama series of the same name was released in 2019, four years after the TV show concluded. Serving as a continuation of the TV series, the big screen adaptation grossed $194 million worldwide against the budget $13 - 20 million.