Sister Monica Joan fears she is “going to pop her clogs” when she discovers she needs an eye operation on Call The Midwife.

The elderly midwife will be seen struggling with failing eyesight when the BBC drama returns.

Actress Judy Parfitt, who plays Sister Monica, said she is terrified when she finds out her cataracts need to be treated.

“Her friends try to cajole her into having the operation and she fights against it,” said the 82-year-old actress.

“She is absolutely panic-stricken and thinks she’s going to pop her clogs.”

The BBC show’s creator Heidi Thomas said: “One thing I’ve really enjoyed is I’ve created a storyline for Judy across the body of this series which is to do with her failing eyesight and that’s given us quite a slow journey across the body of this series, because it’s realistic.

“I love writing about her.”

Call The Midwife returns to BBC One on Sunday.