Losses to the Federal Deposit Insurance Corp. on poor loans it acquired from Continental Illinois National Bank and Trust Co. of Chicago could run to more than $1.5 billion, according to Continental's parent firm.

The loans were acquired in the 12 months ended Sept. 30, 1985, under the FDIC's financial rescue of the bank. Continental Illinois Holding Corp. said Friday that the FDIC's loss on energy-related loans, which amounted to about 53 percent of the transferred loans, "could increase substantially" if oil and gas prices remained at the sharply lower levels reached since last September.

The updated estimate was prepared by Continental Bank in its role as administrator of the transferred loans and contained in a report filed with the Securities and Exchange Commission.

The report estimated that the FDIC's possible loss on the loans through Sept. 26, 1989, could range from approximately $1.16 billion, at a 7 percent interest rate, to $1.56 billion at a 12 percent rate. In the third quarter of 1985, the interest rate on the assumed debt was 7.54 percent.

The estimated loss to the FDIC was far greater than an earlier estimate made last May, which was based on the transfer of loans with a book value of $2 billion on Sept. 26, 1984, the date of Continental's reorganization. The possible loss on those loans was estimated in May to be about $600 million.

Continental said the substantially greater loss estimate was made in part because of "lower estimated collections" on the loans transferred in 1984, plus associated net interest costs on the loans. The new estimate also covers another $477 million in loans transferred by Sept. 30, 1985.

It does not, however, cover another $104 million of loans assumed by the FDIC in the last three months of 1985. At year's end, Continental had an option of transferring $919 million more of poor loans to the federal agency by Sept. 26, 1989.

A spokesman said he couldn't "pinpoint any specific" type of loans that might account for the sharply higher estimated loss but that energy-related loans "obviously were a significant portion" of the total loans involved.

The Chicago bank's financial problems surfaced in July 1982, when federal regulators shut down Penn Square Bank of Oklahoma City. Continental held $1.1 billion in loans originated by the defunct bank, most of which later proved to be worthless. But a large number of other Continental loans also went sour over the next two years, many related to the oil and gas industry.

Under the 1984 bailout plan, the FDIC can exercise an option in 1989 to acquire all of the old common stock of Continental Illinois Corp. held by Continental Illinois Holding, a new entity created during the reorganization, if the agency loses $800 million on the bank's loans.

While Continental Illinois Holding noted that its stock "would likely become valueless," it said the ultimate value of the shares could depend on whether the price of Continental Illinois Corp. increased significantly over the period.

Continental Illinois Corp. reported a net income of $150.5 million for 1985, in contrast to a net loss of $1.1 billion in 1984.