A referendum drive to change Fairfax County's form of government must rely on petitions already filed and will not be allowed to add more signatures, a Circuit Court judge ruled yesterday.

An anti-tax group is seeking the referendum as a way to unseat the current Board of Supervisors, but the county's voter registrar said this week it is not clear whether the group has gathered enough valid signatures to force the matter onto the ballot. The group needs signatures from 10 percent of the county's 405,884 registered voters.

Concerned that they could fall short, Frederick T. and Marcia P. Dykes, founders of Citizens for Sensible Taxation, asked Judge Thomas J. Middleton yesterday for permission to include more signatures in the count.

Middleton denied the request, ruling that under state law, "the petitions must contain the signatures at the date of the filing," and allowing more signatures at a later date "would create confusion."

He said the county's voters were not being deprived of their right to hold a referendum because the measure's sponsors could collect more signatures and present all of the voter petitions to the county again.

James H. Falk Jr., an attorney for the Dykeses, said they probably would not appeal the decision because it would take too much time.

The group hopes to force a special summer referendum to change the county's urban county executive government to a county manager style of government. If the group succeeds, it hopes to unseat the current board in a separate election in November, one year before the board's four-year term expires.

A change to the county manager form of government would abolish the position of an at-large chairman of the Board of Supervisors, currently held by Audrey Moore (D), in favor of a chairman elected by the other supervisors. The county also would lose a variety of powers specifically granted to it by the General Assembly, particularly the broad discretion to bargain with developers to provide public improvements before granting rezonings.

However, time is running out on the group because of deadlines in state codes. Unless all legal questions are resolved and enough signatures are validated by July 6, there will be no chance of a special election, and the board would serve out its term.

The group's ultimate goal is to persuade local and state lawmakers to limit increases in homeowner tax bills to 5 percent a year.

If the current referendum drive fails, it would seem highly unlikely that the Dykeses could refile the petitions with additional ones and complete the entire signature validation process again by July 6. Nonetheless, Falk said, they may try. Another option would be to appeal the entire matter, possibly to the state Supreme Court, but that could also be time consuming.

Even if they overcome those obstacles, two others remain: Some legal experts, including Virginia Attorney General Mary Sue Terry, believe the sitting Board of Supervisors cannot have its term shortened by a change in government, and many believe that a new state law, which takes effect July 1, prohibits a special summer referendum.

Falk said that a new petition would have to be filed before July 1 to avoid the new law.

On April 23, the Dykeses filed about 48,000 signatures. The county registrar's office has discounted about 6,000 because they are duplicates, illegible or belong to people who are not registered voters.

General Registrar Monica J. Horan said she hopes to have a final count by Friday.