Contract talks between the U.S. Postal Service and the nation's two largest postal unions were stalled for a second time yesterday with no further formal negotiations planned before last night's 12:01 a.m. deadline for reaching an accord.

The two unions, the National Association of Letter Carriers and the American Postal Workers Union, submitted a joint wage proposal late Sunday night, but by last night the Postal Service had not responded. Talks were continuing only in small committees handling specific issues.

No disruption of mail service was anticipated because federal law prohibits postal workers from striking.

At a news conference yesterday, Postal Workers Union President Moe Biller dismissed the possibility of an illegal strike. "We don't think that this is the time for it with what's going on in this country -- the presidency under siege, Congress involved, the American people concerned," he said. "We touch every home and every business in this country."

The latest breakdown occurred less than 12 hours after the two sides had returned to the bargaining table after four days in which the unions boycotted the talks. As soon as negotiations resumed, Postal Service negotiators submitted a wage proposal calling for 1.6 percent average salary increases over the next three years.

Vincent R. Sombrotto, president of the Letter Carriers, said he and Biller rejected the offer "in six seconds" and responded with their own wage proposal later in the day.

The unions' offer called for a 4.5 percent average salary increase over three years, 100 percent coverage of cost-of-living increases and a cut in the amount of time it takes a worker to reach the highest salary level. Union officials said they lowered their wage demand from last week's 6.8 percent proposal in exchange for Postal Service concessions on the use of part-time labor.

Sombrotto said the wage issue is the major obstacle to an agreement. "If they show good faith in the wage negotiations . . . , then I don't see why we couldn't settle the other issues fairly easily," he said.

Postal Service spokesman Jim Van Loozen would not comment on the terms of the proposals but said the service was ready to return to the bargaining table. "Both sides have said 'we're ready to talk,' " he said. "But they say the ball is in our court and we've got to make the next move. We're not willing to play that game."

Negotiations broke off last Wednesday, when union officials charged that the Postal Service, in a contract settlement announced the day before with a third, smaller union, had made a secret deal transferring jurisdiction over about 10,000 jobs from the Postal Workers Union to the Mail Handlers Division of the Laborer's International Union.

After three days of informal conversations aided by a federal mediator, the unions and the Postal Service signed a memorandum of understanding on the jurisdiction issue. Both sides agreed that no jobs would be transferred without approval of all unions involved.