The Fairfax Board of Supervisors, facing possible fines and injunctions, has acknowledged it met in secret last December to discuss redistricting but rejected a prosecutor's claim that the meeting was illegal.

The nine elected supervisors submitted a two-page response this week to a lawsuit filed by Fairfax Commonwealth's Attorney Robert F. Horan Jr. Horan charged that the supervisors violated the Virginia Freedom of Information Act when they held an unannounced session in Supervisor Audrey Moore's office on Dec. 11.

The board's response claims the discussion about redrawing district boundaries concerned "legal matters" and so could properly be discussed in closed session. The supervisors did not respond to Horan's allegation that they failed to vote publicly to go into executive session, as required by state law.

The response submitted this week contains the first admission by all nine supervisors that the secret meeting, first disclosed by The Washington Post, did in fact take place. Previously, some had acknowledged they attended the meeting, others denied it and still others declined to comment.

Moore stated earlier that she now believes the meeting was "unwise." Supervisor Nancy Falck said she believes the meeting was proper because she told many of her constituents about it. After Horan filed suit at the urging of Common Cause and the League of Women Voters, the supervisors declined to discuss the case.

The supervisors attended the meeting after months of stalemated discussions on how to bring their districts into line with population shifts revealed by the 1980 census. At the secret session, they reached a compromise that allowed each incumbent to remain within his or her district with a minimum of political damage.

That plan was ratified with minor changes, at a public board meeting the next week.

A circuit court judge can impose fines of $25 to $500 on each supervisor and enjoin the board from future violations if the meeting is found to have been improper. The injunction would mean that future violations would be criminal offenses.