Elton John wants the media to back the hell off of Prince Harry and Meghan Markle -- and warns if the harassment continues they'll end up like Princess Diana.

Sir Elton is livid about various reports and criticisms of the Royal couple going on gas-guzzling private jet trips -- most recently to Elton's home in France -- while advocating for environmental issues.

The famous singer says he's "deeply distressed" over the "distorted and malicious" accounts of the Duke and Duchess of Sussex visiting him last week, and feels the need to protect Harry and his family "from the unnecessary press intrusion that contributed to Diana’s untimely death."

Elton says the couple's recent private vacay at his home was his way of providing them a peaceful break after a hectic year and their dedication to charity ... but it required a private jet ride for security reasons.

Because of Harry's commitment to the environment, Elton says he ensured the flight was carbon neutral -- by contributing to Carbon Footprint (a company supporting renewable energy) -- so the haters can shut it.

Elton insists it's time for the attacks to stop, saying, "cease these relentless and untrue assassinations on their character that are spuriously crafted on an almost daily basis."

As you know, Elton and Diana were very close friends, and clearly ... he's looking after her son like a protective papa bear.