A Fairfax County judge ruled yesterday that all nine county supervisors violated the Virginia Freedom of Information Act when they met secretly last December and he warned that they could be jailed or fined for any future violations.

Chief Circuit Court Judge Barnard F. Jennings held that the Board of Supervisors met illegally in Supervisor Audrey Moore's office Dec. 11 to redraw the boundaries of their election districts. The redistricting agreement they reached, and toasted with champagne, was ratified with minor changes at a public meeting the next week.

"We made a mistake and it's not going to happen again," Board Chairman John F. Herrity said yesterday in his first public statement on the meeting, which was disclosed by The Washington Post. Most other supervisors declined to comment.

The suit was the first in Northern Virginia since the state's Freedom of Information Act was passed in 1968, according to Commonwealth's Attorney Robert F. Horan. The court hearing before Jennings yesterday lasted only two minutes because the board had agreed to admit to a violation. In return, Horan promised not to embarrass the supervisors by calling witnesses and not to ask the judge to impose fines on them, as the law permits.

Jennings did issue an order barring the board from ever again meeting behind closed doors without publicly voting to do so and prohibiting the supervisors from discussing anything in closed session not specifically permitted under the Virginia law. Violations of the judge's order would constitute contempt of court and would be punishable by fines or jail terms, Horan said.

The injunction will remain in effect until the supervisors' current terms end Dec. 31, 1983.

Leslie Byrne, president of the Fairfax League of Women Voters and one of the civic leaders who urged Horan to sue the board, said she believed the suit was effective. "My particular interest was not to punish the Board of Supervisors," she said. "I wanted to make sure that when they held meetings they are whenever possible open to the public."

Byrne said, however, that she was disappointed Jennings did not place the supervisors under an injunction for as long as they remain on the board, in case they are reelected. She also wanted the judge to order the board to keep minutes of its executive sessions.

"It's very difficult to know what goes on in a closed meeting, and that what they say will happen, happens," Byrne said.

Horan, a Democrat who is a close political ally of several of the supervisors, said state law does not require records to be kept of executive sessions. "You're relying on the good faith of public servants," Horan said. "I think they'll be a little bit cautious in light of all this because of the possibility of criminal sanctions."

But Horan said he was surprised to discover during the preparation of the case how weak the state law is. He said the law does not clearly define "legal matters," which may be discussed in closed session, and does not require the board to say which legal cases they plan to discuss in private.

"I never realized just how loose it is," Horan said. "I would hope one of the things that would come out of this would be that they tighten up the procedures for going into executive sessions."

Lilla Richards, president of the Federation of Citizens Associations and another supporter of Horan's suit, said she also hoped the board would follow the spirit rather than the letter of the state law, which exempts several subjects from the open meeting requirement. "It's certainly very upsetting to have supervisors saying things like, 'We've done it before and we'll do it again,' " she said.

Richards was loosely quoting Supervisor Joseph Alexander, who declined comment yesterday except to say his Dec. 14 statement--"It's been done before; it'll be done again"--was "unfortunate." Several supervisors came to regret statements they made about the meeting, including Thomas M. Davis III, who said in January, "I don't think there was any violation, if in fact there was a meeting."

Supervisors said privately that they believed the meeting received more attention than it was worth. Redistricting happens only twice a decade and has to be discussed privately, they said, because trading and discarding precints in public would be political suicide. Board rules require redistricting every five years.

"Their track record is excellent with this one exception," said County Attorney David T. Stitt, who represented the supervisors in court.