Over the weekend, ADAM LEVINE announced that he was LEAVING “The Voice”, effective immediately.

Season 16 finished up last week, but just three weeks ago NBC said Adam WOULD be returning in the fall.  Adam and BLAKE SHELTON were the last of the original coaches . . . both have appeared in all 16 seasons.

So, what happened?

The “New York Post” claims there’s been some friction between Adam and the network.  Apparently, Adam was upset over some changes to the format, including a new rule that ended the guarantee that each coach went to the Live Playoffs with the same number of team members.

Adam’s team didn’t do so hot this season . . . and by the semifinals, he had NO singers left.  The “Post” says he was frustrated that he didn’t have much to do . . . had attitude issues at NBC events . . . and has been “checked out for a while.”

NBC hasn’t commented, but Adam has . . . and he didn’t mention anything about any drama.  In fact, he still has a lot of LOVE for the show.

In a long Facebook post, he said, quote, “It [was] a life shaping experience that will be close to my heart forever.  Thank you NBC, for signing me up.  I’m truly honored to have been a part of something I’ll always cherish for the rest of my life.

“Thank you to every single coach I ever sat in those chairs with . . . and, BLAKE [EFFIN’] SHELTON.  I couldn’t hide my love for you if I tried.  Seriously.  I tried.  Can’t do it.  Our friendship is and always will be one for the books.

“For me, it was time to move on.  What an amazing ride.  Thank you all so much.”

He specifically thanked numerous people involved in the show, including the fans.

NBC didn’t look far for a replacement.  Blake’s girlfriend GWEN STEFANI will take Adam’s chair.  She was a coach on Seasons 7, 9, and 12 . . . and was an advisor on Seasons 8 and 10.

The other coaches will be Blake, KELLY CLARKSON, and JOHN LEGEND.

Of course, it’s possible that Adam just had something come up with his music career, and needed to clear his schedule.  Other coaches have left to tour or record, and then returned in future seasons.

But it’s definitely worth noting that Adam makes his departure sound permanent.