Prince Harry has been told he needs to keep King Charles and Queen Camilla in mind in the future, following his ill-advised snap with German “fake prince” Mario-Max Schaumburg-Lippe.

On Friday evening the Duke of Sussex attended John Travolta’s Living Legends of Aviation gala where he picked up an award. But one moment raised eyebrows, as Harry posed for a photo with Mr Schaumburg-Lippe – who has been accused of cashing in on his royal connections.

In response, a senior German royal has spoken out and urged Harry to “be more careful” and “show caution” in the future.

Prince Alexander Schaumburg-Lippe told Mail Online: “I hope he [Harry] will be more careful in the future and also always keep in mind the dignity of their majesties the King and Queen.”

Mario-Max was adopted at the age of 24 and soon started using the title prince – despite his relatives, including Prince Alexander – asserting that he is not a prince. The royal accused him of “harvesting our name” for fame.

Prince Alexander added: “If Prince Harry read about these things he may exercise more caution in the future, which would be a wise thing to do. In his position, you can’t surround yourself with people without vetting them first.”

Mario Max has been branded the “King of Trash TV” after appearing on several reality TV shows, including Germany’s Big Brother.

When the picture first emerged, Prince Alexander urged Harry to “stay away from this man” and warned that he “cannot stoop lower” than appearing alongside the so-called “fake Prince”.

Mario-Max boasts a string of royal connections on his official website and points out that he’s a distant cousin of the British royals, as he has links to William of Orange.

He claims he has the right to use the prince title because he was adopted by a distant relative of the family.

On Monday Mario-Max took to Instagram to defend his heritage. He shared a copy of his birth certificate which states he is the son of Prince Waldemar and Princess Antonia.

He added: “I feel so sorry for Prince Harry – solely having a chat with him catapulted myself into the world of false, defamatory and hate-post tabloid journalism as well!”

Mario-Max refers to himself as His Highness and a royal and TV Anchor on his social media profile.

Harry and Meghan’s biographer Phil Dampier said about the picture: “If Harry wants to be taken seriously he should not be posing up with people like this ‘German Prince’ who has been on their equivalent of Celebrity Big Brother and been dubbed the king of Trash TV on shopping channels. It’s very sad to see someone who was once a real life royal with so much to offer reduced to this sort of tacky selfie.”