from www.tmz.com – Montana Fishburne believes she’s been duped by prosecutors who she says went back on a plea deal she struck, and her lawyer has now gone to court to try and enforce the agreement … TMZ has learned.

TMZ has learned Montana’s lawyer, Shawn Chapman Holley, filed blistering legal papers in L.A. Wednesday, claiming last June, prosecutors who filed the assault case agreed to allow Montana to enter a rehab program for 90 days, and in return she’d get no jail time.

Holley says Montana, relying on the L.A. City Attorney’s offer, entered a rehab facility, where she’s been since September.

According to Holley, prosecutors did an about-face in November and told her they wanted Montana to serve 120 days in jail.

Now Holley has gone to the judge, asking her to enforce the deal her client relied on.

TMZ has learned … Montana has now decided to spend 180 days in the rehab facility.