Upstairs Downstairs has been recommissioned, with viewers promised a "huge shock" in the new series of the revived period drama.
Six hour-long episodes of the show, which follows the lives of an upper-class family and their servants in their home in Eaton Place, London, will be shown next year on BBC One.
The corporation's drama supremo Ben Stephenson said: "When it returns in the first half of next year, viewers will find everything they loved at Christmas (when the show was revived by the BBC), but with bold new stories that shed a fascinating light on a changing society.
"A huge shock will ensure events at 165 Eaton Place are never the same again."
The show was first shown on ITV.
The announcement came as Jean Marsh, who appeared in both the original and the revived series as Rose Buck, was honoured as "old maid of the year" by the Oldie magazine.
The show's writer Heidi Thomas said: "For me, going back to Eaton Place feels like opening a jewel box - the house is packed with gems. The Holland family and their servants are set for an extraordinary year, and I am thrilled to be their guardian during this momentous chapter."