Downton Abbey is to go up in smoke as series five begins, when a heartbroken Lady Edith accidentally sets fire to her bedroom.

The character, played by Laura Carmichael, will be seen weeping in bed over her secret daughter, whom she gave up when her lover, Michael Gregson (Charles Edwards), mysteriously disappeared.

But a book too close to the fireplace goes up in flames, putting the rest of the house and its inhabitants in danger.

Fans will see the family and servants scramble to get out in time and to save Downton, uncovering some house secrets along the way.

Executive producer Liz Trubridge explained that they had built a replica bedroom to burn at Ealing Studios rather than filming it at Highclere Castle because it was too risky to attempt the scene in its usual home.

She said: "Because it was a stately home we couldn't set fire to it - the owners didn't want smoke damage to their house."

The series opener also sees the cast in 1924 having mixed reactions to the news that they have a Labour prime minister - some of the servants welcome the change, while butler Carson (Jim Carter) and members of the family including Hugh Bonneville's Earl of Grantham worry their world is about to change, especially when the village asks Carson to chair a committee in preference to the Earl.

Actor Allen Leech said his character, former chauffeur Tom Branson, who now lives as part of the family with his daughter Sybbie after his marriage to the late Lady Sybil, would sympathise with those feeling the class system needed a shake-up. But he believed Branson would be unlikely to play a very active part in it.

He said: "It would be impossible for him to go back to the man he was. I think he's not as naive as he was when he arrived last year, but he's still very strong in his views."

Viewers will also see Anna (Joanne Froggatt) and John Bates (Brendan Coyle) struggling with whether to have a child, but she said Anna's reluctance was nothing to do with speculation that Bates could be a serial killer, following the mysterious death of the man who raped her in the previous series.

She said: "I don't think Anna thinks for a moment that Bates is a serial killer. What I'd like to see for Anna and Bates as a viewer is different from what I'd like as an actor, because you like to play the drama, but it's important to have the happy times in between. They get moments of happiness and positivity for them, but it's not an easy journey."

Executive producer Gareth Neame promised more moments as shocking as the rape scene to come in series five: "We always try to have each season right-angle moments you don't expect to happen, so I'd like to think so."

He added: "We hope to be back again next year. We think one year to the next so we'll see."