Prince William County, in an aggressive drive to collect an estimated $4.5 million in back taxes, is threatening to sell homes and land belonging to hundreds of people, many of whom say they purchased the property with assurances that the land was debt free.

"You can't print my reaction - I'm flabbergasted," said Dean N. McDowell, one of the property owners who recently received a letter with this threat:

". . . We are preparing to file suit to have this property sold for payment of all unpaid taxes assessed against this land. . . .After a sale of the land is made, you will forever lose any interest in the property."

County officials are delighted by the results of the campaign. Since Jan. 1, Prince William has collected $3 million in back taxes and has increased its Moody's bond rating from A1 to AA largely because its tax collection rate has risen from about 95 percent to close 99 percent.

But some real estate attorneys and title examiners-the people responsible for making sure there are no old tax bills on newly purchased properties - are furious. They claim that the county clerk of court's office filed required delinquent tax notices only after the county began its collection drive.

"What appears to have happened," said lawyer Wiley F. Russell of Fairfax County, who handles many real estate transactions in Prince William, "is that they located a bunch of unpaid taxes and put them in the file later."

"It's obvious what happened," said John F. Ribble, head of Columbia Title Agency Inc. in Fairfax. His firm is one of the insurance companies that checks the estimated 500 property transactions that are handled every week at the Prince William Court-house in Manassas. "The delinquent notices were put in after the fact."

But courthouse officials say that's not so. "If delinquent tax notices are missed, they're missed by the title examiners," said Court Clerk Charlton E. Gnadt.

"No one would do that (file delinquent notices belatedly)," said Eloise Reed, assistant cashier in the Prince William tax office and the officials in charge of pursuing delinquent accounts. "Tell the examiners to slow down and don't search titles on Monday,c she said, half seriously.

What baffles Gnadt's critics is how scores of examiners-working on land records from all sections of the county and for various employers-could fail to note the delinquent tax notices. The file containing them in the clerk's office is as accessible as all the other documents that are routinely checked.

Sloppy title work could be costly both for the lawyers, who are potentially liable for the thoroughness of their work and title insurance firms, which must pay any unnoticed back taxes on land they have insured. About half the county's homeowners have such policies, officials say.

For individuals without title insurance, the discovery of old tax bills can be costly. Dean McDowell and his wife Mary, for example, have found themselves confronted with $318.09 in taxes and penalties on their 42 acres of land near Nokesville. They purchased the land in 1976 and only recently were notified of the tax bill that was due the couty in 1971, five years before they purchased the land.

"It's obvious there cannot be errors (by title examiners) so widespread," said Eugene Luther of Fried, Fried and Klewans, a Springfield law firm specializing in settlements. "The mistake is being made elsewhere."

Lawyer Russell said he is convinced the delinquency notices were filed belatedly because he remembers searching one title in November 1978 and finding no delinquent tax notice. Yet the property's new owner has recently received a tax bill dating to 1971. A check yesterday showed that the delinquency notice was currently on file-though there was no notation on the notice indicating when it was inserted in the file in the court clerk's office.

The county's no-nonsense drive to collect old taxes has brought hundreds of people to the clerk's office with their checkbooks in hand.

Some owners have asked for, and received, extensions from the county's threat to sell their property on the courthouse steps.

For example, Fairfax County Juvenile Court Judge Thomas A. Fortkort and the other owners of the Oak Grove Estates subdivision received a delay on paying a 1975 tax bill. According to the delinquency file, the Oak Grove Estates partnership, of which Fortkort is a general partner, owed $2,214.54 in back taxes, counting interest.

Under Virginia law a local jurisdiction can file suit to sell any property on which there are taxes due for more than three years. Some lawyers, not to mention recipients-claim the county's letter smacked of the tactics used by collection agencies.

While suits can be instituted on property with delinquent taxes, the process is slow, and some lawyers say it might be years before the threat could be carried out.

While some Prince William officials say the title examiners are to blame, another theory is offered by James B. Robeson of the Manassas law firm-Robeson, Murphy and Robeson-hired by the county to send out the tough-worded letters threatening to sell the properties.

"The county has never made an effort to collect taxes that were so old," Robeson said. "All the notices are going out at once, so all the errors are being noticed at the same time."

"I can't get excited one way or another," said the Prince William County finance director, Garnett Ball. Yet he did recently write a letter to Robeson, Murphy and Robeson suggesting: "I would appreciate your drawing up a letter of a milder tone."