According to new legal docs, obtained by TMZ, Diddy's lawyer Marc Agnifilo claims they just found out Friday night that prosecutors are in possession of attorney-client privileged material that was seized from Diddy's jail cell at the Metropolitan Detention Center in Brooklyn.

Agnifilo claims the privileged materials include Diddy's handwritten notes to his legal team concerning defense witnesses and defense strategies for his upcoming trial in May.

Diddy's attorney says the feds already admitted to possessing notes recovered from Diddy's cell ... and he says the search and seizure violates Diddy's Fourth, Fifth and Sixth Amendment rights.

What's more, Diddy's legal team says the notes were taken from his cell and turned over to the trial prosecutors in his case, who he believes may have used the material in a legal filing to keep Diddy behind bars.

Diddy's attorney is asking for an immediate hearing to get to the bottom of things ... he wants the feds to answer who authorized a search of Diddy's cell, who determined what materials to take, who decided to give these materials to the U.S. Attorney's Office, who decided to give the materials to the trial prosecutors, who decided not to tell Diddy about the seizure, and what exactly was taken.

In the docs, Diddy's lawyer also rips the feds ... saying the search and seizure is "outrageous government conduct amounting to a substantive due process violation."