MNC Financial Inc., the bank company that has financed more construction in the Washington area than any other local financial institution, said yesterday that it lost a record $440 million in 1990 and that one-third of its $6 billion portfolio of real estate loans was in trouble.

The Baltimore-based parent of American Security Bank and Maryland National Bank said the weakening real estate market increased MNC's problem loans by $1 billion in the fourth quarter of last year, pushing up to $1.8 billion the total amount of loans on which repayment is doubtful.

MNC, already considered by analysts to be one of the most financially troubled bank companies in the nation, said it lost $198 million in the fourth quarter, although $66 million of that loss was attributed to factors unrelated to real estate lending. MNC earned $52.1 million (60 cents a share) in the fourth quarter of 1989 and $245 million ($2.84) for the year.

The quarterly and year-end 1990 losses were the largest yet attributed to the region's real estate slowdown and exceed any prior loss recorded by a local financial institution.

Sources said that Washington builder A. James Clark, who had helped guide MNC for nearly 20 years as a director of American Security Bank, resigned as a director of MNC and American Security last week. Clark, who could not be reached for comment, is head of Clark Enterprises Inc., one of the largest general contractors in the United States.

Although MNC continues to have severe loan problems, the company said in a prepared statement that the sale of its prized credit card subsidiary last month generated enough cash to pay off debts and boost the banks' capital, the financial cushion that protects against losses and is a key measure of financial health.

MNC spokesman Daniel J. Finney said that American Security is now in compliance with federal regulators' requests to boost its capital by June 30. He said Maryland National will meet the standards before the summer deadline.

By paying off maturing debt and replenishing capital, MNC has gone far toward resolving its overall financial difficulties in the coming months, analysts said. However, they cautioned that selling the credit card subsidiary, MNC's most profitable business, could prove costly over time.

Analysts said they expect MNC to report further losses this year, in part because profit from the credit card operation is no longer available to offset future loan losses.

"Sure, they meet the capital requirements, but the question is now, in order for them to maintain those requirements, they have to be able to earn money," said David Penn, who follows MNC for Legg Mason and Co., a Baltimore brokerage. "I think that is going to be very difficult for them to do in the next year."

Penn and other analysts said the magnitude of MNC's real estate problems far exceeded their expectations.

"This is in line with some of the most troubled bank companies in New England," Penn said, referring to the region hit hardest by the real estate slump. "They can't keep this kind of increase up for long."

MNC's property foreclosures increased more than ten-fold in 1990 to $415.5 million from $30.3 million in 1989. Finney attributed the jump to MNC's attempts to shift control of troubled debt from the borrower to the lender.

Finney said the fourth-quarter 1990 loss includes a $66 million one-time charge that resulted from the sale of MNC's consumer finance division and fees and commissions paid to consultants for loan quality evaluations.

Finney said the fourth-quarter level of troubled loans reflected management's assessment of the region's high office vacancy rates, plummeting land values and slowing home sales.

MNC stock closed yesterday at $2.62 1/2, down 25 cents. The loss was announced after the stock market closed.