The BBC should have stopped Call the Midwife at the end of the last series, the family of its real-life inspiration has said, as they claim it no longer resembles what really happened.

The family of Jennifer Worth, the midwife who wrote the original books about her experiences in the 1950s East End, said the happy ending of season three would have been the right time to end the story.

Suggesting the BBC adaptation had now strayed too far from the true source material, Worth's daughter Suzannah Hart added executives know they are "onto a good thing" and will persist without its central character.

Worth, who worked closely with the Call the Midwife team but did before the show was broadcast, is played by Jessica Raine in the popular television series.

Her older self is played by Vanessa Redgrave, who provides a 'voice-over' introduction to each episode. Redgrave will return to series four, and will appear as the elderly Jenny in a special Christmas episode.

Raine, who has left the show to pursue her acting career elsewhere, made her final appearance at the end of series three, when she met a young Philip Worth and embarked on a new career away from Nonnatus House.

Her departure means the young Worth will be written out of the show, with programme-makers exploring the stories of other characters instead.

Speaking at the Telegraph Ways With Words literary festival in Dartington, Devon, her daughter and husband, Philip Worth, have now spoken of their own feelings about the show.

The pair have collated a series of moving letters to Worth, published in a book entitled Letters to the Midwife.

When asked whether the family had been "reasonable happy" with the BBC television adaptation, Mrs Hart said: "On the whole, yes.

"I think the first two series yes, because they followed very closely my mother's writing.

"But the third series has strayed quite a lot and because they've used up most of the material Heidi Thomas, the scriptwriter, is now making the material up herself.

"She's doing it very well and very sensitively, she's doing her research, but it's not the same.

"They're doing a fourth series now and I don't think there's anything that's any resemblance whatsoever to mother's writing.

"Initially we had quite good involvement but it seems to have drifted by the wayside."

When asked if she thought the BBC should have ended Call the Midwife when her mother, Jennifer Worth, finished her books, she said: "Yes, in a way.

"They've written Jenny out of the series simply because Jessica[Raine], the actress that played her, doesn't want to come back again.

"In my opinion that would have been the time to end it.

"She was pedalling into the distance with the young Philip Worth who she'd just met, and that would have been the point to finish it."

She added of the corporation: "Anyway, they're onto a good thing so they're going to keep it going."

Philip Worth, Jennifer's widower, said: "The emphasis right from the beginning, when we spoke to the staff of the film company, was not that they should be letter to letter verbatim, but that there should keep the spirit of the books alive.

"The spirit of the nuns and the feeling of the book. We made that point and I think they've kept to it."

Mrs Hart added: "They have kept to it. [But] It's not the same no. We know the books intimately so obviously the stories that are not my mother's stand out a mile. But there we go."

When asked to respond to the comment, Heidi Thomas, the show's writer and executive producer, said: “The medical, social, and emotional stories at the heart of Call The Midwife continue to inspire and compel me. The Nonnatus House nurses and midwives feel like my best-loved friends.”

Ben Stephenson, controller of BBC Drama, added: “Call The Midwife has been a phenomenon that has won the hearts of the nation, it’s one of BBC One’s most successful series enjoyed by millions and we hope it continues for many years to come.

"Jennifer Worth's original books still play a crucial role in inspiring the next series, supplemented by the testimony of other midwives from the era and new historical research.”

Jennifer Worth was heavily involved in the creation of the BBC adaption of Call the Midwife, and personally convinced actress Miranda Hart to join the cast as the perfect 'Chummy'.

The early series are based closely on her experience as a midwife in the London's East End, as detailed in her books Call the Midwife, Shadows of the Workhouse and Farewell to The East End. She died in 2011.

Her widower, Mr Worth, told an audience compiling a last book about her correspondence had helped to "keep her memory alive".

The book, Letter to the Midwife: Correspondence with Jennifer Worth, is out now.

Buy Letter to the Midwife: Correspondence with Jennifer Worth from the Telegraph Bookshop