Princess Diana’s marriage to then-Prince Charles was off to a rocky start, with signs as early as their honeymoon.

Following their royal wedding, which was watched by 750 million viewers around the world, the couple boarded the royal yacht Britannia for a 12-day tour of the Mediterranean.

The distance between the newlyweds was already evident in a letter Charles wrote from the yacht, even though he declared, “Marriage is very jolly.”

“Diana dashes about chatting up all the sailors and the cooks in the galley etc. while I remain hermit-like on the veranda deck, sunk with pure joy into one of Laurens van der Post’s books,” he wrote, revealing that the couple was already spending their time separately during their honeymoon.

Things intensified when Diana spotted Charles wearing personalized cufflinks, a gift from her husband’s ex-girlfriend, Camilla Parker Bowles.

“Cufflinks arrive on his wrists — two ‘C’s entwined like the Chanel ‘C’s. Got it in one; knew exactly. ‘Camilla gave you those, didn’t she?’ ” Diana recalled to writer Andrew Morton in recordings made for his 1992 biography. “He said, ‘Yes, so what’s wrong? They’re a present from a friend.’ And, boy, did we have a row.”

Royal biographer Catherine Mayer also recounts in Divide and Rule that Charles became “in a lather about the loss of a cufflink just before the pre-wedding ball at Buckingham Palace,” which were part of a set Camilla had given him — something Diana was “well aware” of.

The cufflinks weren’t the only hint that there were still feelings between Charles and Camilla. Before the wedding, Diana had discovered a “goodbye” bracelet that the future monarch intended to give Camilla. Then, on the honeymoon, as they compared schedules, Diana saw two photos of Camilla fall out of her new husband’s diary.

As the cruise continued, not only was she lonely and sick — Diana admitted to Morton and in her 1995 Panorama interview that she suffered from bouts of bulimia — the new Princess of Wales was becoming “obsessed by Camilla.”

“At night, I dreamt of Camilla the whole time,” she later told Morton.

After a tumultuous decade of marriage, Charles and Diana separated in 1992 and officially divorced four years later. Both acknowledged relationships outside their marriage, including Charles’s renewed romance with Camilla, whom he wed in 2005, years after Diana died in a 1997 car accident.

Looking back, Mayer says she believes the marriage was doomed from the start, and the honeymoon was the first major warning sign: “The thing that is so striking about Charles and Diana is they got married understanding almost nothing of each other — or what they were doing to themselves.”