THE KID IS NOT HIS SON SAYS MICHAEL JACKSON'S BROTHERS

Michael Jackson's brothers have now come forward ... calling BS on the so-called DNA test proving Brandon Howard is Michael's long lost son.

A rep for the brothers tells TMZ, the brothers are united in scoffing at the claim ... and for a variety of reasons.

First, a source says in Michael's 50 years on earth, he never mentioned anything about having a child other than Prince, Paris and Blanket.  And, other sources who had day-to-day contact with Jackson echo the same to TMZ.

A source also says the brothers met B Howard one time -- back in 2012, when he came backstage for a meet and greet after one of their shows.  He never even intimated he might be a relative.

And the brother's say Joe had a professional relationship with Howard's mom, yet he never breathed a word of this ... and they say he would have said something, especially after Michael died.

The rep calls the so-called DNA test "unfounded hearsay."

We hope another person will not come up with a similar story.