A defendant’s admissions or statements made when tendering a plea cannot be used against him if the plea is withdrawn or rejected and the matter is brought to trial.
According to Rule 12(f) of the Massachusetts Rules of Criminal Procedure,
evidence of a plea of guilty, or a plea of nolo contendere, or an admission, or of an offer to plead guilty or nolo contendere or an admission to the crime charged or any other crime, later withdrawn, or statements made in connection with, and relevant to, any of the foregoing pleas or offers, is not admissible in any civil or criminal proceedings against the person who made the plea or offer.
Notes to Rule 12(f) add that
It is drawn from Fed. Cr. Crim. P. 11(e)(6), although it is broader in scope, since unlike the federal rule, its application to statements made in the course of plea negotiations is not limited only to circumstances where the defendant is negotiating with a government attorney….However, simply expressing a desire to plead guilty to a police officer who does not have any authority to bind the Commonwealth does not bring a defendant’s statement within the scope of this subdivision. Id.
However, such statements can and likely will be used for impeachment purposes if the defendant chooses to testify at trial.