Cellular phone companies must get Fairfax County approval to erect transmission towers within the county, even when they are built on state-owned land, according to a Virginia Supreme Court decision.

The ruling leaves open the prospect that companies could be forced to stop using or dismantle at least a dozen existing towers and to halt plans to construct as many as 18 others in the county.

The court on Friday ruled unanimously in favor of Fairfax County--and against the Virginia Department of Transportation and two wireless phone companies--in the dispute over the construction of cellular transmission towers. The case has implications for other jurisdictions such as Albemarle County and Williamsburg where cell phone towers have been controversial, legal experts said yesterday.

The dozen towers, which range from 80 to 164 feet in height, were built on highway rights of way without the zoning approval of the Fairfax County Planning Commission and despite the objections of neighbors, who said they are unsightly and hurt property values.

Bell Atlantic Mobile and AT&T Wireless had struck deals three years ago with the state Department of Transportation to build about 30 towers--known as monopoles--on rights of way owned by the state along highways in Fairfax. In return, the companies agreed to buy and install highway monitoring cameras, advisory radio systems and emergency call boxes on the sites as part of lease payments totaling nearly $150,000 per tower over 10 years.

But after people complained, the Fairfax County Board of Supervisors sued in September 1997 to stop the companies from operating the towers and prohibit new ones until they had the county's permission. The county argued that the towers, which carry cellular phone, pager and radio signals, are "public utility facilities of commercial entities" and thus subject to approval under its comprehensive plan.

VDOT and the companies contended that because the towers would be equipped with traffic cameras and other devices to promote highway safety and emergency response, they should be considered "shared communications facilities" between the department and the companies. Under a Virginia law that permits state agencies to build certain structures on their property without the approval of local zoning authorities, the defendants argued that Fairfax County had no right to regulate VDOT's use of the land.

The Fairfax County Circuit Court ruled in favor of the department and the companies, but the Supreme Court overturned that decision, finding that the department in effect gave up its exemption from local zoning authority by leasing the land to commercial concerns.

In his opinion, Justice Lawrence L. Koontz Jr. wrote that the companies must submit plans for existing and proposed towers to the county Planning Commission for approval. Under the ruling, the Circuit Court would decide the fate of existing towers should they not be approved.

"Our attorneys are reviewing the ruling," said Department of Transportation spokesman Andy Farmer. "It's too early to make an assessment of the impact on existing and proposed towers."

David P. Bobzien, the Fairfax County attorney, said residents "are very used to having a public hearing process available in any land-use decision, and that's what was at issue here."

"We feel this is a real blow to right of state agencies to use their own property as they deem fit," said Andrea Linskey, a spokeswoman for Bell Atlantic Mobile. "We entered into a good-faith agreement with VDOT to provide excellent wireless coverage for consumers. . . . We're going to evaluate the next step."