The episode will air at 6.15pm on Wednesday 25 December on BBC One.

Guest stars will include Sandi Toksvig as Sister Gibbs, Raymond Coulthard (Emmerdale) as Captain Goodacre and Simon Armstrong as Major Fawcett.

The official synopsis reads: “In Poplar, Christmas 1958 is not just about the opening of presents, but also the closing of deep wounds. Wartime trials are relived – and a new enemy confronted – before peace and love are restored to centre stage. Fairy lights festoon the streets as Jenny’s relationship with Alec continues to blossom and Shelagh prepares for her quiet wedding to Dr Turner.

“Suddenly, Nonnatus House is awoken by a hammering on the door. But this time, it’s not a panicking father, it’s the police. Just around the corner, an unexploded bomb has been uncovered and the district has to be evacuated. Electrical power in the immediate vicinity has been cut off, and it is dark as the nuns and nurses scramble out of the door to join the crowds of evacuated residents scurrying down the pitch-black street.

“As the community struggles to stop Christmas being reduced to ashes, the Nonnatus House staff reach out to those in need and keep spirits high, rekindling the spirit of the Blitz.”

A third series of eight new 60-minute episodes was commissioned earlier this year and will air in 2014.

> Buy Series 1 on DVD on Amazon.

> Buy Series 2 on DVD on Amazon.

Are you looking forward to the Christmas special? Let us know below…