Loudoun County taxpayers failed to pay about $15.7 million in real estate and personal property taxes on time this month, nearly double the amount of taxes unpaid at this time last year.

Only 86.1 percent of Loudoun's taxes were submitted by the Dec. 5 deadline. The 13.9 percent of unpaid taxes was by far the highest amount of outstanding bills since before 1975, said Loudoun Treasurer George Titus.

It is the second year in a row that the county has seen a record amount of unpaid bills. Last year, 91.4 percent of the county's tax levy was paid, leaving a shortfall of $8.7 million, Titus said. At the time, it was the largest amount uncollected since a recession in 1982, he said.

By June, with property owners paying their bills late, the amount of unpaid taxes had fallen to about $2.6 million, still more than twice the $1.2 million shortfall at the same time in 1989, Titus said.

Titus said the delinquencies were likely the result of a slumping real estate market, which has been troubled by overbuilding in the last few years, and a sharp fall in land prices, which were inflated over the years by real estate speculation.

"It's principally in real estate," Titus said. "I'm sure they're having a cash flow problem."

According to county tax records, some of the largest delinquents include developers B.F. and Associates, of Springfield, which owes more than $250,000; Evergreen Mills Investments, of McLean, which owes more than $250,000; and Ashburn Center Partnership, of Chantilly, which owes more than $320,000.

Titus said Ashburn Center Partnership may owe less because it has sold some of its properties since the beginning of the year.

County Administrator Philip A. Bolen agreed that the shortfall is another indicator of the worsening condition of the county's economy, which had been fueled, in part, by rapidly escalating land values. "It clearly is a concern. It clearly is a message," he said. "Because we are dealing with large parcels and expensive land, the tax bills themselves are expensive."

But Bolen said the delinquencies will have a significant impact on the current budget only if they remain unpaid after June, the end of the fiscal year and the time when the next county budget takes effect. He said Titus would boost his tax-collecting efforts. Titus said the county would eventually get nearly all the money through court judgments, tax liens and other collection methods. But delinquencies after June could force the county to borrow money in the short run, which costs taxpayers in interest payments.

Those who owe taxes are encouraged to negotiate a way to make their payments, Titus said, but they must initiate contact with the office of the treasurer.