Jenny Agutter has revealed it was challenging shooting the Call The Midwife Christmas special in the summer.

The actress filmed the BBC One show’s seasonal special during the heatwave in June, which made it difficult as her nun alter-ego was wearing her wimple.

“The thing to remember all the time is cold, cold, cold,” said Jenny, who plays Sister Julienne. “When one’s cold and pretending to be hot, that’s difficult, but when it’s hot you forget that you’ve got to be tightened up – physically, you’ve got to see that.”

The festive episode will be screened on Christmas Day, with series three following in the New Year.

It’s the second time the show has done a seasonal special, and it’s not a calm Christmas for the Nonnatus House team, after an unexploded bomb, a relic from the Second World War, is discovered in Poplar.

There are also many births at the nursing convent, as well as an emotional storyline involving Dr Turner’s son, Timothy, as the doctor prepares to marry Shelagh – formerly known as Sister Bernadette.

Pam Ferris, who plays Sister Evangelina, admitted she cried when she watched the episode. “It’s embarrassing if you’re in the damn thing!” she said. And she revealed she thinks her husband will be in tears when he sees it too, saying: “I’ll watch it with him on Christmas Day and I’ll have the tissues ready – he’s a bit of a weeper!”

As birth is such a big element on the programme, Pam said it is ideal Christmas viewing. “Whether you’re religious or not, it is rebirth, out of the darkness of winter, finding new life. That’s why it is perfect for Christmas,” she said.

The Call The Midwife Christmas Special screens on BBC One on Christmas Day.