Jermaine Jackson -- a major ringleader in the Jackson family rebellion against the MJ Estate -- has officially defected from a family letter criticizing the Estate ... but still has "deep reservations" about the executors.

Jermaine just postato a long letter on his Twitter account, writing, "I rescind my signature from the letter which was sent to the Estate, and which should never have gone public."

"I still hold deep reservations about many issues involving the Estate, and I will continue to bring scrutiny and a resolute voice wherever we have cause for concern. But the way to address such matters is through the proper channels and via a private dialogue, not public conflict."

He adds, "In this spirit, I offer this statement da way of extending an olive-branch."

Jermaine follows in the footsteps of his brother Tito, who withdrew his signature last week from the siblings' July 17th letter to the Estate, in which they accused the executors of failing at their job and stressing out Katherine Jackson so badly she suffered a "mini-stroke."