The Defense Department announced yesterday that the principal contractors in the canceled A-12 jet fighter program can postpone returning $1.35 billion in federal funds to avoid "extreme financial pressures" that might have threatened their welfare.

That figure is $600 million less than the Pentagon had originally told McDonnell Douglas Corp. and General Dynamics Corp. they would have to return to the government for work that was never performed. The payments, however, would not have to be made until the companies' counterclaims against the government are resolved in court.

Pentagon spokesman Pete Williams said the postponement was intended to give the companies -- which were the top two defense contractors in 1989 and are still working on at least 13 other major weapons systems besides the A-12 -- a "breathing space." He said the decision had been requested by the firms and was made "in recognition of the positions that these companies hold."

Williams sidestepped questions about whether the two companies would have failed without federal relief. But he said the decision "is a prudent step to safeguard the nation's industrial base" and that an immediate payment would have affected "the other programs they are working on."

A Pentagon statement said that at the same time the payment was deferred, the companies were informed that they are expected "to tighten their belts financially, to take steps to improve their management of programs ... {and} to carefully control their expenditures and capital investments."

Williams said this was "not a quid pro quo" but instead reflected longstanding government concerns about the way the firms are run. The companies' debt will accrue interest at a rate of 8 3/8 percent while the court claims are being resolved, he said.

The companies laid off more than 3,200 workers after Defense Secretary Richard B. Cheney decided to cancel the $6.2 billion contract to build the first eight A-12 planes last month. Westinghouse Electronic Systems Group, a major subcontractor, laid off 1,200 workers at its plant near the Baltimore-Washington International Airport.

The Navy, which had considered the A-12 its highest-priority new aircraft, had been slated to spend a total of $52 billion for 620 aircraft by some estimates.

But Cheney instead fired several A-12 program managers and ordered the Navy to charge the companies with default because they could not design, develop and test the plane within the required cost and time limits. Cheney also said the companies could not predict how much it would cost to correct major problems with the plane, which was to incorporate "stealthy" materials -- ones that do not reflect radar waves -- enabling it to slip more easily past Soviet detection systems after being launched from U.S. aircraft carriers.

The companies have denied they defaulted on the contract and said they did not owe the $1.9 billion initially claimed by the Pentagon. They have counter-sued to recover $1.4 billion of their own costs.

Williams said the new, lower estimate of $1.35 billion reflected "a more thorough checking of records as we enter into the legal phase of this." But he and other officials could not explain what records are now believed to be in error.

McDonnell Douglas has already taken a $219 million after-tax charge on the program, while General Dynamics took a pretax charge of $450 million. General Dynamics yesterday reported a loss of $530 million in the fourth quarter of 1990 compared with a profit of $82.8 million a year earlier.