Unisys Corp., the struggling defense systems and computer company, yesterday reported an unexpectedly hefty quarterly loss and said it will lay off 5,000 more workers and accelerate plans to sell off pieces of its business in an effort to lessen its debt burden and return to profitability.

The company also announced the resignation of chairman W. Michael Blumenthal, the former Treasury secretary who engineered the 1986 merger of Burroughs Corp. and Sperry Corp. into Unisys. Blumenthal, who also resigned as a director, had passed the chief executive title last April to James A. Unruh, who will now assume the chairman's post.

Yesterday's announcement of a $357 million quarterly loss -- far higher than Wall Street had expected -- was just the latest spate of bad news at a company whose bleak fortunes recently have prompted speculation that the firm might file for bankruptcy.

Analysts have said Unisys does not face an immediate cash crisis, and Unisys officials have consistently denied that it is considering seeking protection from creditors in bankruptcy court. The company now has $2.6 billion in outstanding debt.

"The rumors are irritating but not true," Vice Chairman Curtis Hessler said in an interview yesterday. But, he said, "We are in a difficult business environment and difficult capital environment. We're also over-leveraged."

Hessler would not rule out the possibility that the company's defense operations, which accounted for $2.3 billion of sales last year, might be among those slated to be put on the block under the stepped-up divestiture program announced yesterday.

But one investment banker familiar with the defense industry said it could be difficult to sell the defense lines while the company remains under the cloud of a federal investigation of Pentagon contracting fraud. Although several former executives have pleaded guilty to criminal fraud charges, no charges have yet been brought against the company.

On Wall Street yesterday, the news had little effect on already-depressed Unisys shares. The stock closed down 12 1/2 cents at $3.37 1/2.

The 5,000-person layoff announced yesterday will probably have only a limited impact on the firm's 2,700 employees in the Washington area, Hessler said.

As the largest seller of commercial computer systems to the government, Unisys holds key contracts that span government agencies, from the Internal Revenue Service to the Pentagon. Last year, Unisys won the government's largest personal computer procurement, for up to 250,000 PCs from the Pentagon.

The company also sells the Pentagon a broad range of electronics equipment used in planes and ships.

Overall, the company said, it is facing soft sales in the United States and an unexpected turndown in business in Europe. Recently, company executives announced with great fanfare a broad-ranging strategy to update its aging line of computers. But yesterday the company conceded that it may continue to lose money through the end of the year.

The $357 million third-quarter loss was less than the $648 million loss logged during the same quarter a year ago, but still far worse than Wall Street had expected. Although the loss included a one-time restructuring charge of $181 million associated with the layoffs, analyst Stephen Cohen of SoundView Financial Group in Stamford, Conn., said the company continued to do poorly even on an operational basis despite a slight increase in revenue.

The restructuring "wasn't expected but it was something they needed to do," said David Schofield, an analyst in Chicago with Duff & Phelps. "When Blumenthal put the company together, he expected that {it} would be a $20 billion company by 1993, and they're still only halfway there. Clearly, they'd been planning on more growth than they've been getting and it was time to admit that," Schofield said.

Yesterday's announcement comes on the heels of a spate of bad news for the company. In the past few weeks, Unisys suspended its common stock dividend, and Moody's Investors Service downgraded the company's credit rating. Standard & Poor's followed with its own downgrade yesterday.

The new layoffs, to be completed over the next few months, come on top of a 12 percent cutback that has trimmed the work force by 10,000 people over the past year to 75,000.