Just as Xerox Corp.'s blissful courtship of Loudoun County was heading toward a happy marriage, Leesburg threatened to disrupt the ceremony by contesting who should serve parts of the Xerox development with water and sewer service.

Loudoun County is scheduled to vote Dec. 16 on Xerox Corp.'s application for rezoning a 2,267-acre site just east of Leesburg so that it can go ahead with Potomac Park, a collection of corporate office parks and a residential community that Loudoun officials are lauding as its showpiece development.

But the town and the county are fighting over who should run utility lines to the western part of the site, and some members of the Leesburg Town Council are threatening to take the issue to court.

An age-old political rivalry between the town and the county appears to be underlying the dispute.

Leesburg has been miffed in the past when Loudoun County has stepped on its municipal toes, and now the town -- which is growing rapidly in size and local importance -- wants to retaliate, county Supervisor Andrew R. Bird III said.

County Supervisor Frank I. Lambert said the wounds go even deeper. The water and sewer issue has reignited the county's long-held fear that Leesburg will annex more county land and eventually seek city status, robbing the county of some of its tax base, he said.

But Brian T. Kelley, a member of the Leesburg Town Council, called that view "paranoia" on the part of the county board. The town has more than enough land to cope with since it annexed seven square miles in 1983, and simply needs the extra utility customers to help pay its huge water and sewer debts, he said.

Leesburg today charges relatively high utility rates because it has a $7 million debt to pay off on its water plant. The plant was expanded in 1983 to a capacity of 3.99 million gallons a day, more than three times Leesburg's current needs.

Town and county officials are meeting in executive sessions to try to smooth out the problems.

One point that no one seems to dispute is that, if Leesburg extends its utility lines eastward, it would open for rapid development the vacant farmland lying between Goose Creek and the Leesburg bypass which was brought into the town limits by the 1983 annexation.