Set in 1927, the big-screen instalment of the hit ITV drama, created by Julian Fellowes, continues after the series left off in 1925 in the 2015 Christmas special.
The drama filmed at Yorkshire locations including Harewood House in Leeds.It will see the occupants of Downton Abbey, both above and below stairs, thrown into disarray when it is announced that the King and Queen of England are to pay a visit to the house.
Dame Maggie, Dockery and Bonneville all reprise their roles, alongside Allen Leech, Dame Penelope Wilton, Joanne Froggatt, Phyllis Logan, Elizabeth McGovern, Jim Carter, Laura Carmichael and Lesley Nicol.
Also attending the glittering event at the Cineworld Leicester Square will be cast newcomers Tuppence Middleton, Stephen Campbell-Moore and Imelda Staunton, alongside Lord Fellowes and the film's director, Michael Engler.
Actor Kevin Doyle has said he modelled an absurd curtsy he performs in the Downton Abbey film on former prime minster Theresa May.
The actor, who plays footman Joseph Molesley in the TV series and in the new big screen version, told a press conference: "I modelled the bow/curtsey on Theresa May.
"She was meeting some royal or another and it was such a preposterous curtsy I thought 'I'm going to have that'."
Downton Abbey is released in UK cinemas on September 13.