Princess Diana’s private letters to acting legend Terence Stamp are finally coming to light — and they reveal a surprisingly cheeky, vulnerable, and mischievous side of the late royal that the public rarely got to see.

The newly uncovered correspondence shows Diana joking about Prozac, sending flirty adult-themed postcards, and leaning on the acclaimed British actor during one of the most painful chapters of her life.

The Princess of Wales, who died at age 36 in a Paris car crash in 1997, struck up a warm friendship with Stamp in the early 1990s. At the time, Diana was under intense pressure as her marriage to then-Prince Charles unraveled behind palace walls.

Stamp, who died last year at age 86, clearly held onto the keepsakes from their bond. Now, a collection of letters, postcards, and personal items from his estate is set to be auctioned by Bonhams on June 15 as part of its Sound & Cinema sale.

The letters offer a rare look at Diana away from the stiff royal image the world often saw. Instead, they show a woman who was funny, sharp, emotional, and desperate for trusted friends during a deeply turbulent time.

One insider familiar with the correspondence said the letters expose “a side of Diana that the public rarely got to see.”

“There is humor, warmth, and a level of trust that speaks volumes about the friendship she shared with Terence,” the source said. “What stands out is how comfortable she felt discussing personal matters while also showing a playful and mischievous sense of humor.”

Another insider said the messages reveal a bond built on respect and understanding.

“Diana clearly valued Terence as someone she could speak openly with at a time when she was under enormous pressure,” the insider added.

The correspondence dates back to the early 1990s, around the same period Andrew Morton’s bombshell biography Diana: Her True Story was preparing to blow the lid off the princess’s private misery inside the royal family.

In one October 1991 letter, Diana thanked Stamp for lunch and for his support. She wrote that she was “touched to the core” by his understanding of her royal role and everything that came with it.

Then came the eye-catching line.

“Three cheers for Prozac, not the American variety I hasten to add!” Diana joked.

One month later, she wrote to Stamp again after another lunch, thanking him for serving caviar before adding another playful remark about her well-being.

“No Prozac withdrawals as yet!” she quipped.

But the most eyebrow-raising items in the collection may be the postcards Diana sent to the actor.

In December 1991, the princess mailed Stamp a cheeky postcard featuring a bare female breast with the caption “All The Breast from London.”

Another card thanked him for champagne during what she called a “lunch a la yeast.” The card showed a cartoon couple in bed under the caption, “Why did God invent sex?” Inside was the punchline: “So that married people do something at least twice a year.”

The risque jokes are a far cry from the polished, proper image Diana was expected to maintain in public. But those close to the collection say they show how she used humor to cope with the crushing scrutiny around her.

At the time, Diana was living under the glare of constant headlines, palace tension, and a marriage that was quietly collapsing. Her private messages to Stamp suggest she found moments of relief in trusted friendships far from the royal machine.

The letters are part of a larger auction honoring Stamp’s remarkable life and career. Claire Tole-Moir, head of popular culture at Bonhams, said the sale will “showcase the many sides of Terence Stamp” and highlight his relationships with several famous figures, including Jean Shrimpton, Julie Christie, and Princess Diana.

The auction will also include personal photographs and costumes tied to some of Stamp’s most memorable roles, including The Adventures of Priscilla, Queen of the Desert, Far from the Madding Crowd, Last Night in Soho, and Wall Street.

Stamp’s estate said the collection reflects the “full breadth of the man: the actor, the writer, the style icon, the traveller, the seeker.”

“Terence lived with great intention,” the estate said. “Everything he surrounded himself with was chosen for a reason, whether a finely made suit, a piece of art, or a letter from a dear friend.”

For royal watchers, however, the most fascinating pieces may be Diana’s own words — funny, intimate, and startlingly human.

Decades after her death, the letters offer yet another reminder that behind the palace gates and global fame was a woman trying to laugh, survive, and find people she could truly trust.