Virginia's top prison official warned yesterday that the state's soaring inmate population is causing severe crowding requiring new prison construction that could ultimatley cost more "than taxpayers can reasonably sustain."

A report released by Corrections Department Director Terrell Don Hutto blames the relatively stiff sentences given by the state's courts for the increase in inmates. Virginia is among the top 10 states in the nation is terms of the average sentence length - about 2 1/2 years - and has one of the nation's top 13 incarceation rates, Hutto said.

The report, called "Options for the 80's," calls for sweeping reforms in the state's sentencing practices to reduce the prison population of offenders convicted of nonviolent and minor crimes.

Without such reforms, Hutto said, prison officials foresee a 35 percent increase in the nimber of inmates at the state's 42 institutions by 1985, from the current 9,500 prisoners to 12,865.

Hutto warned that the increase, combined with a series of recent court decisions that bar overcrowding, will leave the state prison system more than 8,000 beds short by 1990.

He estimated that building enough prisons to meet the project increase in inmates and still fall within court guidelines on overcrowding would cost $453 million if the prisons were built now. He noted that those costs will probably rise sharply with inflation.

"Construction costs are becoming so excessive that new prisons to meet commitment increases under existing practices would incur more expense than taxpayers can reasonably sustain," the report says.

The state's overcrowding problem is already so serious that 8,300 inmates are packed into 2,445 cells, most designed for one or two prisoners, the report says. In many cases, as many as 10 to 15 prisoners share a single toilet or wash basin, according to hundreds of complaints filed by inmates in federal courts.

Although 3,000 beds have been added to the system since 1975, the inmate population has jumped from 5,000 five years ago, and 1,200 inmates awaiting space in state prisons are being held in county and local jails.

"It causes a backup here," said Fairfax County Sheriff James D. Swinson, who has had to farm out 66 of the 90 state prisoners in his backlog to holding cells in other counties.

Among the reforms called for in the report, which Hutto termed a master plan for the state prison system, are work-release programs, halfway houses and sentences under which persons and sentences under which persons convicted of nonviolent crimes are ordered to pay restitution to victims while working outside prison walls.

Hutto cited one recommendation that offenders be ordered to perform unpaid community service instead of serving time as "a particularly attractive option."

He also praised the report's call for incresased use of probation and parole. If the average inmate stay in prison were cut from 28 months to 24, the prison population would be reduced to 7,640 by 1985, a decrease even from the current figure, he said.

The study's recommendations and Hutto's support for them drew immediate support from Virginia Attorney General J. Marshall Coleman, who earlier this year recommended legislation proposing similar reforms . The proposed legislation, along with several similar bills, was defeated by the General Assembly.

"He's on the right track," said Coleman. "With this crowding problem, we could tip the corrections system over the consitutional brink." Coleman said the state's sentencing practices leave "jails full of bad-check writers with no room for bank robbers. There's really no law on sentencing. It's absolutely a no man's land."

Hutto said his department would suggest legislation to nex year's General Assembly to revise sentencing practices and seek funding for at least two new 500-bed prisons at and estimated cost of $27 million each.

The report, directed by a committee chaired by Virginia Public Satety Director Selwyn Smith, noted that 48 percent of Virginia's inmates have been convicted of nonviolent crimes, and that speeding up their eligibility for parole would empty cells and save money. According to Corrections Department figures, it costs $7,565 to keep an inmate imprisoned for a year, but only $670 to supervise a parolee.

"Sentencing criteria, whether in statutes or in departmental of judicial guidelines, should emphasize the principle of the least drastic alternative," the report said.