Even the most conservative estimates of Virginia's prison population indicate the state could need two new prisons by 1990, the staff of a legislative committee said today.

Gary Henry, chief methodologist for the state's Joint Legislative Audit and Review Commission, told the panel that the state's prison system could top 11,100 by 1990. That is about 2,000 less than projected by the state Department of Corrections.

Some legislators disputed the report. "I don't think we're going to need them new prisons ," said state Sen. Edward E. Willey (D-Richmond) chairman of the Senate Finance Committee. "A lot can happen in five years.

"I'm not sure these projections are accurate. And I'm not sure we don't have enough facilities to take care of it."

The staff report put the capacity of Virginia's prisons in the year 2000 at 9,791 -- with another 1,300 beds of "reserve capacity," a bureaucratic term for double-bunking in two prisons and filling up space in the dormitory wards of field units and main prisons. In June, Virginia's prisons housed 9,787 inmates.

The commission staff recommended several ways for finding more prison space, including building new prisons, enlarging existing ones, expanding community programs and placing a limit on inmate populations.

If the administration of Gov. Charles S. Robb decides it wants more prisons, the General Assembly will have to make the money available this session or next. It would take easily four or five years to plan, design and build a new prison.

Last October, the Corrections Department discovered a major flaw in its projection methods that led to an overestimate of some 2,500 prisoners in 1990.

Then the staff of the House Appropriations and Senate Finance committees turned up with different figures that affected the budget and the opening of a new prison in Nottoway County in southside Virginia. All that led to the legislative commission's mandate to look into the method of projecting the prison population.