What is a conflict between a defendant's attorney and his former client?

Such a conflict springs from the attorney's duty to provide effective assistance to the defendant facing trial and his fiduciary obligations to the witness with whom he has or had a professional relationship. (Leversen v. Superior Court, supra, 34 Cal.3d at p. 538, 194 Cal.Rptr. 448, 668 P.2d 755.) "An attorney is forbidden to use against a [present or] former client any confidential information ... acquired during that client relationship. [Citations.] Moreover, the attorney has a duty to withdraw, or apply to a court for permission to withdraw, from representation that violates those obligations. [Citation.] So important is that duty that it has been enforced against a defendant's attorney at the instance of his former client (who was also a codefendant) even at the expense of depriving the defendant of his choice of counsel. [Citation.]" (Ibid.) In a word, a conflict based on the attorney's obligations to a criminal defendant and to a present or former client, "as well as conflicts arising out of simultaneous representation of codefendants, may impair a defendant's constitutional right to assistance of counsel." (Ibid.)