Somerville, N.J. ? Jayson Williams could walk on a legal offensive foul.

The ex-Net’s manslaughter trial was thrown into jeopardy Thursday after it was revealed prosecutors had failed to turn over potentially critical evidence about the shotgun at the heart of the case.

The bungle opened the door for defense lawyers to seek a mistrial or even “the ultimate sanction — dismissal of the indictment,” Williams’ attorney Joseph Hayden told Judge Edward Coleman.

Under New Jersey case law, a mistrial granted for prosecutorial misconduct could carry a finding of double jeopardy, noted Billy Martin, another lawyer for Williams.

That would mean the former basketball player could not be retried on charges he recklessly killed limo driver Costas (Gus) Christofi early Feb. 14, 2002, when he flipped up his Browning 12-gauge shotgun and it discharged.

Martin accused prosecutor Steven Lember of suborning false testimony and prosecutorial misconduct — charging him with waging an unrelenting, vindictive “campaign to deny Mr. Williams a fair trial.”

In addition to seeking a mistrial and dismissal of the charges that could put Williams behind bars for 55 years, the defense could ask to reopen its case to present new witnesses and/or permission to recall its gun experts to the stand.

The startling development came a day after the defense rested without calling Williams to testify on his own behalf.

The evidence at the center of Thursday’s legal wrangling includes typewritten notes and 23 photos from a Feb. 5, 2003, examination and partial disassembly of the shotgun conducted by prosecution rebuttal witness Larry Nelson, a Browning Arms Co. executive.

Lember informed the defense of the material’s existence late Wednesday — just hours before he planned to call Nelson to counter defense claims that debris in the 10-year-old gun, coupled with wear and tear, could have caused it to discharge without a trigger pull.