Researchers have discovered parts of Anne Frank’s diary previously hidden, including ‘dirty’ jokes and references to sex.

The entries, written on 28 September 1942, while the 13 year-old was in hiding from the Nazis, were covered in brown paper to mask four jokes about sex that Anne described as “dirty” and an explanation of women’s sexual development, sex, contraception and prostitution.

“I sometimes imagine that someone might come to me and ask me to inform him about sexual matters,” she wrote. She also refers to menstruation, describing it as “a sign that she is ripe” but that "one doesn’t do that [have sex] of course before one is married".

Men however answer to a different standard. She wrote: "All men, if they are normal, go with women, women like that accost them on the street and then they go together. In Paris they have big houses for that. Papa has been there."

Although her diary was strictly private and she never intended for anyone else to read it, the fact that she covered these musings up shows the typical embarrassment of a teenage girl. However experts have said that the excerpts reveal more about Anne’s development as a writer than it does about her interest in sex. She addresses a fictional reader, ‘Kitty’; Ronald Leopold, executive director of the Anne Frank House, said that this “is really interesting and adds meaning to our understanding of the diary.”

“It’s a very cautious start to her becoming a writer,” he said. “It’s still very early stages.”

Researchers from the Anne Frank Museum, the Institute for War, Holocaust and Genocide Studies and the Huygens Institute for the History of the Netherlands photographed the pages, backlit by a flash, and then used image-processing software to decipher the words, which were hard read because they were jumbled up with the writing on the reverse sides of the pages. The technology that allowed researchers to look at the covered pages without destroying them wasn’t available until now.

“Anyone who reads the passages that have now been discovered will be unable to suppress a smile,” said Frank van Vree, the director of the Netherlands Institute for War, Holocaust and Genocide Studies. “The dirty jokes are classics among growing children. They make it clear that Anne, with all her gifts, was above all also an ordinary girl.”

“They bring us even closer to the girl and the writer Anne Frank,” said Mr Leopold.