In the bleakest assessment yet of its financial picture, International Harvester Co. warned yesterday that it could be forced to file for protection under federal bankruptcy law even if its latest financial restructuring plan is successful.

"The company's prospects for survival are in substantial doubt," Harvester said in a registration statement filed with the Securities and Exchange Commission yesterday in connection with the restructuring plan.

Discussing several circumstances that could torpedo the company, Harvester cautioned, "There can be no assurance that the company will not seek relief under the federal bankruptcy code in early fiscal year 1983 or thereafter."

A Harvester spokeswoman said the company expects to meet an important deadline today by securing unanimous approval by its 200-plus lenders of a $4 billion restructuring plan. Some lenders had reportedly been balking at the plan, which includes converting some of the company's debts into securities. Without the agreement from the lenders, Harvester will be in default on its loans and could be forced into an immediate bankruptcy filing.

But yesterday's SEC filing indicates the restructuring, which includes concessions from suppliers, apparently will only buy time for the troubled company, which expects to lose $1.6 billion in the fiscal year ending Oct. 31.

Harvester said that while the refinancing plan will keep its net worth from falling below zero by Oct. 31, the company's liabilities could become greater than its assets early in the new fiscal year.

The document elaborates on grim statements the company made about its future in a proxy statement released last week to notify stockholders of a special meeting Oct. 28 to consider provisions of the restructuring.

"If at any time the company's financial and market forecasts appear to indicate that there is no longer a reasonable prospect for the success of the company's restructuring strategy , the company would reconsider its decision to restructure its financial affairs outside the federal bankruptcy code," the company said in the filing.

Harvester's forecasts have been consistently over-optimistic in the past few years, the company acknowledged in the SEC filing, which notes the "consistent failure" of IH's internal marketing forecasts. Analysts say Harvester's truck and tractor markets show no signs of recovery from their three-year slump.

Under the new agreement, creditors would allow the company to keep operating until its negative net worth reaches $200 million. Analysts say IH would effectively be bankrupt once net worth became zero and the company conceded in the SEC filing that negative net worth could be debilitating.

The company said the restructuring would give it a slightly positive net worth as of Oct. 31, compared with a net worth of $824 million on July 31. The company's mounting losses have eroded its worth.

Harvester said it has no hope of regaining profitability in the next fiscal year and said its survival depends on a "significant recovery" in its markets and market-shares in 1983 and 1984.

"The company's internal forecasts show that any significant failure of the markets for the company's principal products to recover in the manner assumed by the company under its strategy for survival, or any significant failure by the company to maintain or recover market share in the manner assumed by the company under such strategy would result in the company not having sufficient cash to continue in business," according to the filing.

"The company has reached the point where it has substantially exhausted the possibilities of further actions to raise cash," the filing continued, noting the sale of several operations in recent months. "Accordingly, if there occurs a significant downward deviation from the assumptions made by the company under its strategy for survival, the company will likely be forced to seek relief under the federal bankruptcy code."

The latest IH restructuring plan is the third attempt to bail out the company. The first disintegrated in September 1981, just before its scheduled approval, when it was disclosed that Harvester had underestimated its loss figures. A second restructuring was put into place last December, but has since been modified twice to fit the company's worsening financial condition.

A Harvester bankruptcy would mean the downfall of one of the Midwest's industrial giants, an old-line maker of farm equipment that dates its history back to Cyrus McCormick's invention of the reaper a century ago. The company has been crippled by the slumping economy and the lingering effects of a six-month strike against it by the United Auto Workers three years ago.