Loudoun County's form of government is outdated and unable to accommodate change in the fast-growing locality, according to a citizens commission that recommends that the chairman of the Board of Supervisors be elected by the voters countywide.

The bipartisan Loudoun Blue Ribbon Committee, in a strongly worded and politically charged report to be delivered to the supervisors today, was created by the County Board in December after state Sen. Charles L. Waddell (D-Loudoun) introduced a bill, now a law, to allow citizens to petition for a November 1990 referendum on electing the board chairman at large.

Waddell and others say that if the Board of Supervisors does not vote soon to hold such a referendum, a citizen petition drive would accomplish that. Prince William County voters approved such a ballot issue in 1988 after a citizens group there pursued the initiative.

In recent months, critics from both political parties have labeled the Loudoun Board of Supervisors short on leadership and effectiveness. They note that the board has been deadlocked 4 to 4 for four months on where to build a new county government administration complex and only recently has formally gauged public support for the $40-million-plus project.

Currently, the eight Loudoun supervisors are elected from their own districts, and once a year they select a chairman from their ranks. The entire board faces election to new four-year terms in 1991, after census data and partisan politics guide a redrawing of district boundaries.

"Put your seat belt on; we're going to have quite a ride for the next 18 months," said William Mims, chairman of the Loudoun Republican Committee and a member of the Blue Ribbon Committee.

Betty W. Tatum, a Democrat who represents the Guilford District in heavily populated eastern Loudoun, is serving her third one-year term as board chairman. Shesaid yesterday she had read only part of the report but is concerned that the panel may be offering the chairman too much power. "What would the rest of the board members be elected for?" Tatum asked. There are five other Democrats and two Republicans on the board.

During the winter session of the Virginia General Assembly, Waddell tangled with Tatum and other Loudoun officials over several issues, most notably a successful effort in the assembly to prevent Loudoun and Fairfax counties from downzoning land in the Route 28 corridor. Waddell and members of the Blue Ribbon Committee say their proposals are not aimed at current county officials.

The citizen referendum drive "is a loose-knit campaign just ready to be triggered" if the Board of Supervisors does not act soon on a referendum for an at-large chairman, Waddell said.

The citizen committee's report, which emphasized the need for one "strong voice for the county," was approved 11 to 0 Wednesday night. Noting that Loudoun's population is expected to double between 1980 and 2000 and referring to the county motto, "I byde my time," the panel titled its report, "No Longer Can We Byde Our Time."

"In Loudoun County not too long ago, the Board of Supervisors could function as a part-time group of citizens who met a few hours a month to direct the day-to-day operations of county government," the report says.

"The practices that worked well before will not be equal to the exigencies of the future. Business as usual will not work," it continues, calling for "a larger, more costly, more formal, more professional government" but stopping short of saying that County Board seats should be full-time jobs.