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Prince Harry is defending his use of private jets, saying that on the rare occasions he might opt for one ... it's only for the well-being of his family.

The Duke of Sussex and his wife, Meghan Markle, have received some heavy criticism lately for cruising around in gas-guzzling private jets ... they took them at least twice in August.

At a presentation in Amsterdam Tuesday -- where PH was discussing a new environmental project within the travel industry called Travalyst -- he told reporters that he actually spends 99% of his life travelling the world by commercial plane.

However, he went on to say ... "Occasionally there needs to be an opportunity based on a unique circumstance to ensure that my family are safe. It's generally as simple as that."

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You'll recall ... Elton John went to bat for Harry and Meghan when they came to visit him in the South of France, backing up Harry's words here by saying their private jet use was necessary for security reasons. The couple also flew private to Ibiza shortly before that.

For Harry, he says balance is the key with his private jet use -- adding that if he has to use a private flight, he works to offset his CO2 output (without getting specific). He says he wants to make that a regular thing in tourism and other travel in general.