Warner Amex Cable Communications, the firm that operates the 15-year-old cable television system in Reston, plans to launch a major expansion of its service next week, a move that could lead to another confrontation over who should provide cable service to that rapidly developing portion of Fairfax County.

The first confrontation over the issue ended last spring, when Warner and Fairfax County agreed to drop a series of lawsuits in state and federal courts over the issue.

The county government, which initiated the suits, said that Warner could not continue to operate a cable system without a franchise and noted that it had awarded a countywide franchise to another firm, Media General Cable of Fairfax Inc. Warner refused to apply for a franchise, arguing it had operated for years without one under an agreement with Reston's developers.

The expansion, which will add 2,000 households to the 7,000 homes served by the Reston system, is the first since the lawsuits were dropped.

Fairfax has set a new deadline of Jan. 1, 1986, for any unlicensed cable operators to apply for a franchise from the county.

Warner officials acknowledged yesterday that the expansion may set off new lawsuits, particularly if the firm again refuses to apply for the franchise. "I suppose it could" go back to court, said Richard M. Berman, executive vice president and general counsel for Warner Amex. He said the firm has no plans to apply for a franchise before the new deadline.

"One thing that's going to happen is that we're going to build our expansion in the interim," Berman said. He called further litigation "expensive, time-consuming and adversarial."

Fairfax granted the cable franchise for all of the county in 1982 to Media General Cable. Company officials were unavailable for comment yesterday.

Along with the county, Media General was named as a defendant in a suit filed last year by Warner Amex in U.S. District Court in Alexandria. When that case was dismissed, Warner Amex appealed to the 4th U.S. Circuit Court of Appeals in Richmond.

Since the firm and the county agreed to drop the litigation, no resolution of the legal issues has been reached.

The county maintains that its cable television ordinance, passed in 1980, requires all cable operators to have a franchise. By granting a franchise, the county has the power to regulate cable service and hold the cable firm to an extensive set of standards and requirements.

"We feel that preexisting contract rights can't be subsequently impaired," said Berman. "We're confident we have the right to be there, this is a good business decision to serve more customers in Reston."

Warner Amex is a joint venture of American Express Co. and Warner Communications Inc. It is the sixth largest cable operator in the country, serving 1.1 million customers.

The firm's Fairfax subscribers live, for the most part, in the southern part of Reston, south of Dulles Access Road, and in the north-central part of the planned community, between the Access Road and Baron Cameron Avenue (Rte. 606). The expansion will include 11 scattered neighborhoods, most in the northern part of Reston.