Princess Diana’s risqué jokes to a well-known film star have been revealed for the first time after letters were uncovered ahead of an auction.

The late Princess of Wales also spoke to British actor Terence Stamp about her use of the antidepressant drug Prozac in correspondence published ahead of the Bonhams auction on June 15.

Stamp – known for his roles in Superman II, Wall Street and The Adventures of Priscilla, Queen of the Desert – died last August at the age of 87.

In one of the letters sent from Diana to the star, dated in October 1991, she thanked him for her “perfect” risotto lunch, and gushed that she was “touched to the core by your understanding of my job/role and what comes with it!”

She also wrote: “‘Not many people would take the time and trouble to delve into such a complex situation … Three cheers for Prozac, not the American variety I hasten to add!”

Three months later, she sent a joke postcard featuring a bare female breast with the caption “All the Breast from London” along with a request for a lunch catch-up.

Another gag card she sent featured a couple in bed with the caption “why did God invent SEX?”, with the answer inside: “So that married people do something at least twice a year.

Diana then wrote her own message, thanking Stamp for the champagne at a “lunch a la yeast”.

The letters are owned by the estate of the late actor and are due to go under the hammer on June 15 as part of the Sound & Cinema collection.