Overly ambitious estimates of the ease with which Xerox Corp. could attract corporate buyers to its proposed multibillion-dollar commercial and residential development in Loudoun County have led to delays in the project, Xerox and Loudoun officials said yesterday.

The delays have frustrated some Loudoun officials, who said yesterday they were told by Xerox last year that major progress was "just around the corner."

Potomac Park, which as advertised would be one of the largest developments of its kind in the Washington area, has bid to become headquarters for 13 major corporations. Also planned for the 2,267-acre site along the Potomac River near Leesburg are 1,800 houses, a resort hotel and conference center and a golf course.

When the Loudoun Board of Supervisors approved a massive rezoning for the development last December, Xerox officials told the county they expected to announce some of the corporations lured to the site within short order, Supervisor Andrew R. Bird III (R-Sterling) said yesterday.

"They said that they would say who the first three or four corporate buyers were within a matter of weeks," Bird said.

Xerox spokesman Thomas C. Abbott said no promises about the timetable for Potomac Park were ever made to Loudoun officials.

So far, no corporate buyers at Potomac Park have been announced, and groundbreaking for the project has been delayed until this fall, Philip Ricciardiello, the Xerox vice president in charge of the project, confirmed yesterday. A "concentrated marketing effort" to attract corporations to the development will not begin until next year, Ricciardiello said.

Xerox's new timetable for Potomac Park coincides with Ricciardiello taking responsibility for the project from Benjamin Knupp, who resigned from the corporation.

"There is no fundamental change in the plan or the direction of the development," Ricciardiello said, adding that "interest remains very high" among potential corporate buyers.

However, he said, it is unrealistic to expect that corporations would buy into Potomac Park until construction of roads and sewers of the project is complete. "I'm a realist when it comes to selling property . . . . We have competition."