Warner Amex Cable Communications of Reston, an unsuccessful bidder for Fairfax County's cable television business, has filed a petition with the Federal Communications Commission seeking to delay the franchise winner's plans to wire the county.

The filing, which argues that Media General of Fairfax County Inc. should be denied a crucial license for microwave transmission because it is facing a court challenge by another unsuccessful bidder, touched off strong criticism from Media General and served to escalate the war over a county franchise that could be worth $260 million in 15 years.

"This simply adds to the effort on behalf of the unsuccessful applicant to delay construction on the cable system by any possible legal device," said Gerald W. Estes, Media General's senior vice president for cable television. "These actions . . . represent a great disservice to the citizens of Fairfax."

Media General and county officials expressed concern that the action by Warner Amex, which currently supplies cable TV service to 7,600 subscribers in Reston, will slow construction of the cable system by several months and possibly longer.

Media General, a subsidiary of the communications company that owns the Richmond newspapers, had planned on starting construction this spring. It has pledged to begin service as soon as this summer in some parts of the county.

Spokesmen for Warner Amex denied the firm is mounting a new competitive offensive for Fairfax's business. "We wanted to point out that we are the authorized cable operator in Reston, and so we brought to the FCC's attention the fact that Media General's authority is under a cloud by virtue of the litigation that has been filed," said Richard M. Berman, a senior vice president of Warner Amex Cable Communications Inc. "It's not a big deal."

But there was immediate speculation within cable circles that the filing marked the first volley in an expected fight by Warner Amex, which does not hold a government franchise for the area it serves in Fairfax, to hold on to its Reston business.

The county's franchising regulations hold that unfranchised cable systems must cease operations after the county's franchise is awarded. Warner Amex's Reston system has been operating for more than 12 years under the terms of contracts it signed with the community's developers.

"It sounds like you've got a real Mexican standoff," said William H. Johnson, deputy chief of the FCC's mass media bureau. He said the Warner Amex filing could delay by up to six months a decision that would ordinarily be unchallenged.

In the petition filed with the FCC Friday, Warner Amex charged that Media General is not eligible for a microwave transmission license because it is still facing a lawsuit by Fairfax Telecommunications Inc., a local partnership which participated with communications giant Tele-Communications Inc. of Denver in an unsuccessful bid for the county's cable business.

A county motion to dismiss that lawsuit, which alleges that Media General officials unfairly "conspired and cooperated" with county officials to win the franchise, will be heard April 19 in Fairfax Circuit Court.

Without an FCC-approved microwave license, Media General officials said, they would be unable to transmit programming from two 300-foot antennas to so-called "hubs" around the county, from which programming would be sent via cable to subscribers.