Crown Princess Mette-Marit has asked Norwegian media to leave her eldest son alone.
Mette-Marit said Marius Borg Hoiby, her son from a previous relationship who has no royal title or official duties, has in recent years "been exposed to pressure" from some media "that I don't feel is worthy of them".
He "is not a public person", she added.
Norway's future queen made headlines in 2001 when she married Crown Prince Haakon, because she was a single mother who had lived a free-wheeling life with a companion with a drug conviction.
The royal website posted the open letter a day before Borg Hoiby's 20th birthday on Friday.
He lives with the popular royal couple and their two children, and now plans to travel abroad like many young Norwegians.