A judge agreed to dismiss allegations by one of three accusers in the case, Lucia Evans.
In an expose published in The New Yorker a year ago, Evans accused Weinstein of forcing her to perform oral sex when they met alone in his office in 2004 to discuss her fledgling acting career.
Manhattan District Attorney Cyrus Vance Jnr has already been fiercely criticised for declining to prosecute Weinstein when an Italian model accused him of grabbing her breasts in 2015.
Weinstein appeared in Manhattan Criminal Court on Thursday morning as Judge James Burke made a decision to dismiss count six of the eight-count indictment against the 66-year-old film producer.
The woman, prosecutors said, told the detective in February that Weinstein had offered them money to flash their breasts during the restaurant encounter. They initially declined, but Evans later told her she had gone ahead and exposed herself to the film producer in a hallway.
The detective allegedly hid information about the witness in dealing with prosecutors and allegedly cajoled the witness into not disclosing what she knew.
The letter said the detective told the witness she had no reason to cooperate with prosecutors and "less is more".
Prosecutor Joan Illuzzi-Orbon insisted the rest of the case is strong and said the district attorney's office was looking into the possibility of bringing additional charges.
The revelation of numerous allegations about Weinstein, beyond the three women involved in the criminal case, helped spark the #MeToo movement against sexual harassment and sexual assault.
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More localised surface water and river flooding is also possible for high ground parts of the south-west of England on Friday and Saturday.
"Sexual assault is a serious crime", Brafman said outside the courthouse in Lower Manhattan Thursday, "but falsely accusing someone of sexual assault is also a serious crime".
"We know of no evidence refuting our client's report that Harvey Weinstein forcefully sexually assaulted her", she said in a statement. "It only speaks volumes about the Manhattan DA's office and its mishandling of my client's case".
He also claimed a police officer had corruptly attempted to influence the case by keeping a witness from testifying about her misstatements.
Brafman, Weinstein's lawyer, told the judge he believes Evans lied both to the grand jury and to The New Yorker about her encounter with Weinstein.
Weinstein's lawyers and prosecutors had been wrangling over the part of the indictment pertaining to Evans in recent weeks in closed-door meetings and sealed court papers.
"I think this grand jury is irreparably damaged".
Brafman identified the detective in the case as Nicholas DiGaudio.
The 66-year-old father of five, out on a $1 million bail and forced to wear a Global Positioning System monitor, attended the latest court hearing and denies any non-consensual sexual relations.
Mr Weinstein has been accused of sexual misconduct by more than 70 women.