Richard Gere and Sylvester Stallone once battled over Princess Diana at a dinner party hosted by Elton John — after Stallone’s plans to romance the royal were derailed by Gere, the singer dishes in his new memoir. “Straight away, Richard Gere and Diana seemed very taken with each other,” John writes in “Me,” according to an excerpt published by the Daily Mail. “She was separated from [Prince] Charles by this point, and Richard had broken up with Cindy Crawford. They ended up sitting in front of the fireplace together, locked in rapt conversation.” Stallone seemed to eye the pair jealously, recalls John, who hosted the dinner party with his now-husband, David Furnish, at their UK home for Disney chief Jeffrey Katzenberg. John was writing songs at the time for 1994’s “The Lion King,” and Katzenberg requested the princess’ presence at the party. As the party moved to the dining room, Stallone and Gere were nowhere to be found. Furnish found them squaring off in a hall “apparently about to settle their differences over Diana by having a fistfight,” John writes. After dinner, Diana and Gere resumed their flirting, prompting Stallone to storm out. “I never would have come if I’d known Prince f- -kin’ Charming was gonna be here,” Stallone snapped, according to John. “If I’d wanted her, I would’ve taken her!” the actor added. Diana, meanwhile, was “completely unruffled.” “Maybe she hadn’t realized what was happening. Or maybe stuff like that happened all the time,” John writes. The Rocketman also dishes on Queen Elizabeth II — who, he says, once slapped her nephew Viscount Linley across the face repeatedly at a party after he refused to watch over his sister. “When he repeatedly tried to fob her off,” John recalls, “the queen lightly slapped him across the face, saying, ‘Don’t’ — slap — ‘argue’ — slap — ‘with’ — slap — ‘me’ — slap — ‘I’ — slap — ‘am’ — slap — ‘THE QUEEN!’ ” Linley did as he was told. When the queen noticed John had seen the exchange, she winked and walked off, he says. “I know the queen’s public image isn’t exactly one of wild frivolity, but … in private she could be hilarious,” John writes. The memoir is slated for an Oct. 15 release.