The Falls Church City Council voted unanimously last night to withhold its share of funds for continued subway construction unless Metro guarantees that the planned West Falls Church station does not temporarily become the end of the subway line.

The latest plan of the Washington Metropolitan Area Transit Authority would make the West Falls Church station the last stop on the Orange Line from 1984 to 1985, and the council doesn't want the heavy commuter traffic a terminus would bring. In 1985, the line would be extended to Vienna.

The council's decision could conceivably block continued Metro construction, since all Washington area jurisdictions must subscribe to a joint financing plan before the 101-mile rail system can be completed.

But Falls Church has until Sept. 15 to ratify the financial agreement, and Mayor Harold Miller said he hopes to use last night's action as a means of winning from Metro a promise not to use the West Falls Church station as a terminal point.

"We have some time to work this thing through," Miller said. "That's why I had it on the agenda tonight and not two weeks from now. The next step will be up to WMATA."

Miller said he fears that if the station were made the end of the line, heavy vehicular traffic would pose a hazard to the more than 600 students at nearby George Mason High School. "I see a real jeopardy for this community if we sign this agreement," he said.

Metro spokesman Cody Pfanstiehl said that "again and again through history" local governments have waited to sign agreements "until the last minute because they want to get a concession."

Councilman Gary Knight expressed the view that Metro had left the coucil little chocie. "They think they have the upper hand," he said. "Who are we but a fly on the back of the Metro elephant?"