4:28 PM PT -- Turns out, Michael Jackson's estate doesn't think these drawings being attributed to MJ are the real McCoy -- in fact, they say they have reason to believe otherwise.

A rep for the estate tells us they do not accept these sketches as authentic -- noting they examined the artwork years ago and upon further inspection, felt even more dubious. They say they've relayed this to the auction house since then, and insist they haven't gotten any evidence that, to them, would suggest these drawings were done by the late icon.

Michael Jackson may be best known for his legendary singing career, but he was also quite the sketch artist ... and his signed drawings are about to be available for purchase.

The singer's rare collection -- which is made up of 78 sketch drawings of various images, including the likes of Marilyn Monroe, Queen Elizabeth II, Peter Pan, Andy Warhol, etc. -- will be auctioned off by Kings Auctions Beverly Hills partnered with Kings Auctions Las Vegas on August 3. The location has yet to be disclosed.

The auction will reportedly first welcome bids for the entire collection ... though, a $1 million opening offer is required or the auction will move to selling the pieces separately.

Kings Auction tells TMZ ... they came into possession of the artwork via recent court proceedings. Prior to their acquisition, the pieces of art were stored at a private art space in Santa Monica, Calif. -- where MJ often visited to work while recovering from plastic surgery procedures.

We're told Michael's signature on the drawings has been authenticated by Roger Epperson -- who also confirmed the sketches were not prints.

This isn't the first time MJ's personal belongings have been auctioned off. Remember, years after Michael's death in 2009, his possessions from his last residence in Los Angeles -- including furniture, paintings, and antiques -- were sold off.

In fact, Kim Kardashian has purchased a number of the late King of Pop's belongings. She notably bought MJ's velvet jacket and white fedora hat ... which she gifted to daughter North West.

Anything for a little piece of music history, right???