George Santos is warning the Bureau of Prisons ... predicting an "international nightmare" if he's brutalized behind bars during an upcoming prison stint.

The former New York congressman tells TMZ ... if anything bad happens to him in federal prison, the BOP is going to have a problem on their hands.

Santos warns ... if his bodily autonomy is violated, it's going to be bad for the BOP. He imagines his attorney releasing a statement about him being brutalized or sexually assaulted in prison ... something he doesn't think the BOP wants to deal with.

George notes ... he's not one to shut up either.

He's due to report to federal prison July 25, and he tells us he's scared of the big house and fears for his life.

George worries ... "What's going to happen to me?"

He adds ... "We know in theory the BOP is for the well-being of inmates, but we know in practice a lot of s*** happens to inmates. I don't want to be a BOP statistic."

As we reported ... Santos was sentenced to 87 months after pleading guilty to wire fraud and identity theft charges.

George tells us he's hoping to be placed in protective custody ... and he says his attorneys are working on the request ... along with requests for a potential pardon, clemency and commutation.

Despite his concerns about life in prison, George says he's not learning any self-defense tactics. He tells us he's nonviolent and doesn't want to resort to self-defense.

In the meantime, George says he's laying low and spending as much time with family as possible while he's still on the outside ... and he admits his head is "all over the place" and he doesn't know what to think as he waits to surrender.