Members of the Fairfax County Board of Supervisors threatened yesterday to block construction of the cross-county Springfield Bypass if the route approved Thursday by Virginia highway officials is not altered.

"If the highway commission does not immediately modify its alignment, I will move to have it stricken from the Fairfax County master plan," said Supervisor Marie B. Travesky (R-Springfield), whose district would include the major portion of the 35-mile, $180 million state highway.

Travesky said that if the road is not included in the county's master plan, it would be ineligible for federal funding. Without federal aid, which would account for as much as 70 percent of the project's cost, state officials are unlikely to press for construction of the road, county officials said.

John F. Herrity, the chairman of the Fairfax supervisors and a 10-year champion of the Springfield Bypass, said that unless changes are made in the proposed route "I will seriously rethink my support" for major portions of the road.

Warned in advance of the state's route, Herrity disclosed that he has written a letter to Virginia Gov. John N. Dalton, a fellow Republican, asking for a meeting on the controversial raod.

Fairfax officials are upset by changes the State Highway and Transportation Commission made in the route, which had been recommended unanimously by the county supervisors. The major alterations in that plan occur near a proposed government center west of Fairfax City., at Fort Belvoir and at the Newington interchange with I-95.

State highway officials said the changes were made to better serve traffic and to save an estimated $18 million in construction costs.

The Springfield Bypass, which also is known as the Springfield Parkway and the Reston Bypass, has been creating controversy since it was proposed in the early 1970s. Supporters of the road contend that it is needed to relive traffic congestion and provide easier access to growing portions of the county.

Opponents long have charged that the bypass actually would create congestion by encouraging more bedroom communities and commercial growth. Throughout the debate there have been charges that developers were manipulating the proposed road route for financial gain.

Some state highways officials dismissed the threatned veto of the bypass by Fairfax supervisors as a bargaining tactic. Fairfax has lobbied for the road too hard and long, they said, to "chuck it" over route changes.

"We agree thoroughly with the need for the road," said James Yeatts, a design engineer for the highway department. "But I'm sure you're been hearing how scarce highway funds are . . . . If Fairfax County backs off of this, there are a lot of other needs in Northern Virginia."

But Supervisor Travesky sounded militant.

"If the highway commission has set out to devise a route that could never be built, they have succeeded admirably," said Travesky. "Why they have chosen to ignore the wishes of the citizen's committee as well as the Board of Supervisors can only be speculated upon."