EXCLUSIVE

Things are about to get uglier for 'Real Housewives of New York' star Jules Wainstein -- her estranged husband is planning to file for full custody of their 2 kids ... TMZ has learned.

Sources close to Michael tell us his attorney is prepping docs asking a judge for primary custody ... He also wants an order limiting Jules to supervised visitation of their 2 young children.

This, of course, comes on the heels of Jules calling 911 Wednesday morning after the 2 engaged in a nasty argument. As we reported ... Michael left before cops arrived. No one was arrested.

We're told Michael will claim Jules is just too erratic to have the kids.