You knew it was coming … and now, so does Prince Harry — a multi-million dollar offer to get down and dirty in a XXX movie … courtesy of the biggest porn studio on the planet.

Vivid Entertainment honcho Steve Hirsch just fired off a letter to the royal palace in London — offering Prince Harry $10 million to star in a big budget adult film called “The Trouble with Harry” … thanks to his recent naked romp in Vegas.

According to the letter, the film would also star “little Harry” and would feature some REAL royal carousing — in the form of full-on boning. 

Hirsch writes, “We assure you the sex will be well-scripted, and the crown jewels will not be ‘minimized’ in any way.”

God save the peen.