The Federal Deposit Insurance Corp. has lost $1.3 billion -- nearly twice original estimates -- on its bailout of Continental Illinois National Bank since 1984, it said yesterday in its first disclosure of losses connected with the bank's near collapse.

The federal agency said it also lost $600 million on last year's record 120 bank failures and that losses from bank failures in prior years were $400 million higher than expected.

The FDIC, which insures deposits up to $100,000, has added $2.3 billion to its loss reserves to reflect the anticipated losses from bank closings and the Continental crisis, the biggest government-assisted bailout ever.

The increased costs are the freshest sign of the escalating pressure the FDIC feels from the Continental Illinois crisis and increasing small bank failures. FDIC's growing problems were underscored by its announcement that, for the first time in 35 years, it will not refund unused insurance premiums to member banks.

Despite the increased losses, the FDIC ended 1985 with $17.9 billion in reserves, a $1.4 billion increase over 1984, it said. That means the federal fund has $1.22 for every $100 in deposits it insures, a fund-to-deposit ratio it has maintained since 1980.

"The insurance fund has grown," said FDIC Chairman L. William Seidman. "The FDIC's financial statements will show a solid insurance fund."

The $1.3 billion loss on Continental matches the bank's own estimate of losses that would be absorbed by the federal fund. The loss could grow much higher as falling oil prices erode the value of the energy loans the FDIC purchased from the ailing Chicago bank under a 1984 bailout agreement.

The announcement of FDIC's losses come as pressure mounts on Congress and regulators to allow banks suffering from bad energy, farm and foreign loans to stretch out loan write-offs or use other special bookkeeping techniques.

Bank industry sources say that, at Senate Bank Committee hearings next week, Seidman is expected to offer his own proposal for keeping troubled banks afloat.

The $1.3 billion Continental loss includes $700 million in losses in 1984 and $600 million in 1985, the FDIC said. The agency's 1984 financial statement will be adjusted to reflect the change.

The 1984 statement also will be adjusted to show a $400,000 loss increase on 230 banks that failed before 1985, most of them in 1984 and 1983, the FDIC said.