The D.C. Court of Appeals yesterday reversed the armed robbery conviction of a 28-year-old Washington man because a D.C. Superior Court judge forbade him to talk to his lawyer during a lunch recess.

In a 6-to-3 opinion, the appeals court ruled that Judge Nicholas S. Nunzio "deprived [the defendant] of his constitutional rights to assistance of counsel."

It is the fourth time in a year that Nunzio has been reversed by the city's highest court.

The appeals court ruling overturned the 1975 conviction of Reginald A. Jackson, who was found guilty of holding up a Safeway cashier at gunpoint. The appeals court ordered a new trial for Jackson.

During the trial, Jackson had testified on direct examination but had not been cross-examined by the prosecutor when Nunzio called a recess for lunch.

The court record shows that Nunzio told Jackson, "Since you're under oath, sir, at this point, you discuss nothing with anyone, not even with your lawyer because he's finished with you. As of now, you don't discuss your testimony with anybody, you understand?"

The majority court opinion, written by Judge George R. Gallagher, said Jackson was "deprived of counsel's guiding hand during a critical stage of the proceedings."

Last August, a three-judge appeals court panel said in upholding the conviction that while Nunzio was wrong in his admonition to Jackson, it was "harmless."

Yesterday the full nine-member court said "deprivation of counsel"s assistance is presumptively prejudicial and . . . inherently constitutes plain error."