In a statement, police said the rape “relates to (a) sexual act without intercourse” and that the victim “was unable to resist the act.”
According to local reports, the Norwegian palace declined to comment on Mr. Hoiby’s latest arrest.
Mr Hoiby was arrested by police at 11.12pm (10.12pm GMT) on Monday in a car with the victim in a separate incident.
He faces charges of assault and criminal damage in connection with that separate incident on August 4.
The police said, external on Tuesday that the charges were expanded to include “intimate abuse” and “other violation” of a restraining order against the same victim.
They added that the report was drawn up for driving a vehicle without a driver’s license.
Police also searched Mr Hoiby’s residence in Skowgum, close to where the crown prince and his wife live.
Mr Hoiby has also previously been accused of assaulting ex-girlfriends Juliane Snekestad and Nora Haukland, as well as threatening a man.
