(TMZ) - Miss Delaware Teen USA Melissa King has officially given up her crown today ... after a porn surfaced on the Internet that allegedly features the 18-year-old beauty queen.

A rep for the Miss Teen USA pageant just released a statement, claiming, "I would like to confirm the Miss Delaware Teen USA pageant has received a resignation letter from Miss King’s attorney."

King reportedly denies it's her in the hardcore sex footage -- but the girl in the video looks and sounds EXACTLY like King.

In the vid -- shot some time in June 2012 -- the girl addresses the camera, claiming she turned 18 a couple months beforehand in March, which is King's birth month. The girl also describes her favorite sex position ... before the actual sex begins.

King was crowned Miss Delaware Teen USA in November ... four months after the sex footage was shot.

A rep for the pageant tells us, they have already reached out to the first runner-up Hailey Lawler in order to replace King.