The Supreme Court voted 8 to 0 yesterday to grant constitutional protection to criminal defendants who plead guilty because of misunderstand, duress, or misrepresentation by their own counsel.

Georgetown University law professor Sherman L. Cohn told a reporter that "hundreds and probably thousands" of state prisoners across the country now may seek hearings in federal courts with the claim that their lawyers had misled them about the plea bargains that led to their incarceration.

To cure this problem in the future, Cohn said, states that have not yet done so can pass laws similar to a rule in many federal courts that requires prosecutors, defense counsel and defendants to spell out fully - in open court - any plea bargain that may have been made.

For decades, plea bargaining - which lightens punishments of defendants while conserving the resource of judges and prosecutors - "was a sub rosa process shrouded in secrecy and deliberately concealed" by participants, including judges, the court said in an opinion by Justice Potter Stewart.

Not untii 1971, when the court ruled plea bargaining "an essential component" of the riminal process to be encouraged when "properly administered," were "lingering doubts about the legitimacy of the practice finally dispelled," Steware wrote.

Only 37 days after that ruling, while North Carolina still was operating in what Stewart termed "the atmosphere of secrecy which then characterized plea bargaining generally," authorities arraigned Gary D. Allison on robbery charges.

He pleaded guilty to one count after his court-appointed lawyer promised him that he would get the minimum sentence of 10 years and instructed him to deny to the judge that a promise of a 10-year sentence had been made.

The judge then read 13 questions from a printed form. In reply to one, Allison said he understood he could get 10 years to life: in reply to another, he denied anyone had made any promises to induce him to plead guilty.

The judge accepted the plea by signing the form, thereby certifying that Allison had been fully advised of his rights and had pleaded "freely, understandingly and voluntarily."

Three days later, at a sentencing hearing of which no record exists. Allison got 17 to 21 years. The Fourth U.S. Circuit Court of Appeals held that he was entitled to seek relief in the federal courts and sent the case back for a hearing The Supreme Court affirmed.