A couple on a Delta flight to Hawaii is accused of having too much fun on their way to paradise … ’cause other passengers claim they tried to join the “Mile High Club” while in their seats, and now authorities are investigating.

Here’s the deal … a passenger onboard Delta Flight DL312 from Minneapolis to Honolulu tells TMZ he saw a woman performing oral sex under a blanket on a man seated near him … right there in the middle of the packed economy cabin!

The passenger we spoke to was so horrified, he recorded some of it, and it sure appears something lewd is taking place.

Law enforcement sources tell us the FBI and deputies at Honolulu’s Daniel K. Inouye International Airport responded when the plane landed — so, the alleged activity was reported from the flight crew to the ground.

We’re told no arrests have been made yet, and standard protocol would be for the U.S. Attorney’s Office for the District of Hawaii to review the case and determine whether to charge the couple.

Delta declined to comment and referred us to law enforcement. We reached out to the US Attorney’s Office for more info … so far, no word back.