In a blow to the credibility of one of the Square's most popular bars, a state commission has denied an appeal by the Crimson Sports Grille to revoke a six-day suspension issued for underage drinking.

The Alcohol Beverages Control Commission (ABCC) ruled that the Grille knew or should have known that it was serving an underage drinker when the Cambridge Licensing Commission conducted a sting last October.

Within the next 30 days, the Grille will have one more opportunity to appeal the ruling, this time to superior court.

But if the decision sticks, it will be the second time the Grille has been cited for underage drinking.

Richard V. Scali, executive director of the Cambridge Licensing Commission, said he was pleased with the ruling, and warned that the Grille would be in trouble unless it reformed its ways.

"If you're out to serve under-age people and you're not monitoring the door and you're not monitoring your bar, you're gonna get caught," said Scali.

"It's very uncommon that they even get a second offense," Scali continued. "This place has been blatantly in violation for the past two years. They're right up there with the worst offenders in the city."

The Grille's owners did not return several phone calls yesterday.

The ABCC's ruling appeared to ultimately turn on the credibility of the student cited for underage drinking, Joseph D. Deal '98.

When the Cambridge Licensing Commission conducted their sting on October 14, chief investigator Andrea Boyer identified Deal's drink as a rum and coke. Deal testified before the ABCC that the beverage contained no alcohol.

The ABCC ruled that Deal's testimony was "not credible," particularly since the student admitted in court that he entered the Grille using a fake ID.

But the ABCC's decision went further. The commissioners ruled that the Grille "knew, or should have known, that Deal was under the age of" 21.

But the ABCC's finding that Deal used a fake ID to enter the Grille could be significant if an appeal is filed, according to the Grille's lawyer, James J. Rafferty.

"If this person came on the premises with a false driver's license, there is a defense available for the licensee," Rafferty said. "If the licensee can demonstrate that they reasonably relied upon a Massachusetts driver's license, he's entitled to a defense that he used due care."

Deal refused to comment on the ruling.

Scali, however, seemed confident that the ruling was appropriate.

And Scali repeated his longstanding threat to recommend revocation of the bar's liquor license if the Grille is brought before the Cambridge Licensing Commission again.

Scali said it is unusual for an establishment to be charged so many times.

He also said he would continue to investigate and prosecute underage drinkers across the city, and particularly in the Grille.

"My investigators in the police department will be out in full force and will be monitoring the Crimson [Sports Grille] as well as any other bar in Harvard Square," said Scali. "We will be prosecuting each and every underage person in the court system and they will not get just community service. We are looking to get the $300 fine enforced as well as a possibility of 90 days [in jail].