EXCLUSIVE

"Girls Gone Wild" founder Joe Francis is the star of a brand new hardcore sex tape ... problem is, he claims it was STOLEN from his longtime GF ... and now he's threatening legal action, TMZ has learned.

The footage -- shot several weeks ago -- features Francis getting down and dirty with his girlfriend Abbey Wilson -- who (deservedly) won the GGW "Search for the Hottest Girl in America" contest back in 2012.

We know ... the footage contains material from several sexual liaisons ... and it's currently being shopped to various media outlets.

Francis tells TMZ ... he has NOTHING to do with the tape's release -- and, ironically, he's trying desperately to stop it from going public by THREATENING the person who has it.

Joe claims the footage was ripped off of Abbey's iPad -- which was recently stolen from her bag at LAX.

Francis' lawyer, David Houston, tells TMZ, "It is not only unfortunate, but it is a crime. As such, this office will take all necessary steps to determine who in fact has done this and who is attempting to distribute the video."

Joe's attorney says they have already contacted the authorities -- and want to deliver a strong warning to whoever has the footage:

"When we catch you, we will see that you are prosecuted to the fullest extent of both the criminal and civil laws."