Whitney Houston Almost KICKED off a Flight for NOT wanting to wear her seatbelt!

Multiple sources tell us ... Whitney boarded a flight in Atlanta Wednesday afternoon ... and when she settled into her seat, a crew member asked her to buckle up.

We're told Whitney refused and "got diva" on the crew member ... until another crew member came over and warned Houston that if she didn't buckle up, she would have to get off the plane.

Whitney eventually allowed one of the crew members to grab the buckle and fasten it for the singer -- and the plane was then cleared for takeoff.