Fairfax County prosecutor Robert F. Horan Jr. said yesterday he is investigating whether county supervisors violated Virginia's open meeting law when they gathered secretly last month to redraw the boundaries of their election districts.

The nine Fairfax board members met without public announcement in Supervisor Audrey Moore's office on Dec. 11 to discuss the politically sensitive redistricting that is required after each census--and then toasted each other with champagne after they reached a compromise. Their plan, which insures that no two incumbents will be placed in the same district, was ratified unanimously with a few minor changes at the board's next public session.

Horan said yesterday he undertook his investigation at the request of Common Cause, a citizens' lobbying group with about 2,500 members in Northern Virginia. Two other influential Fairfax groups, the Federation of Citizens Associations and the League of Women Voters, will decide next week whether to ask Horan to sue the board, their presidents said.

"If such a meeting took place, it's clearly a flagrant violation of the state law," said League president Leslie L. Byrne, who said she learned of the session through news accounts. "It would be a case of lawmakers becoming lawbreakers."

The Virginia Freedom of Information Act forbids county boards from meeting in private except when they vote in advance to go behind closed doors to discuss matters such as litigation, personnel, or real estate transactions.

Willful violations of the law are civil, not criminal, offenses and can be punished with fines of between $25 and $500, which supervisors must pay out of their pockets. The fines go to the state Literary Fund, which supports schools and libraries. Horan said that if he believes the law was broken he can also ask a judge to order the board not to repeat the offense, in which case future violations would become criminal offenses punishable with jail terms.

Board Chairman John F. Herrity yesterday declined to comment on the meeting. Other supervisors could not be reached or said they did not think they violated the law. Some have declined to acknowledge the meeting occurred.

"Clearly, we all know what the answer's going to be," said Supervisor Thomas M. Davis III. "I don't think there was any violation, if in fact there was a meeting."

Nancy K. Falck said she did not believe the meeting was secret. "I don't know what the other supervisors did, but I informed a number of citizens in my district that a meeting was going to occur," Falck said, "and it was at their urging that I attended."

Moore declined to discuss the legality of the meeting, pending the results of the investigation, which Horan said would last "a couple of days."

"I guess we'll just have to see what he says," Moore said. "I think in retrospect it wasn't wise."

Lilla D. McC. Richards, president of the federation that represents about 100 neighborhood civic associations, said the board published two possible redistricting plans on Nov. 16, held a public hearing on them on Dec. 7--and then voted for a third, previously undisclosed plan, on Dec. 14.

"By closing the meeting at which this carving up was done, they effectively prevented people from objecting to the changes," Richards said. "For every neighborhood that was restored as a result of that secret meeting, as many others were chopped up with no opportunity to respond."