Postmaster General William F. Bolger held fast to his refusal to return to the bargaining table yesterday amid continued efforts by federal mediators to avert an illegal nationwide mail strike threatened for tomorrow.

Despite a federal court order banning a walkout, leaders of the two largest postal unions gave no sign of backing away from their unions' mandates for a strike if the Postal Service refuses to reopen contract talks.

The National Association of Letter Carriers faces a strike deadline of midnight tonight, while the American Postal Workers Union has a deadline of Wednesday night.

After asserting his willingness to talk with the union leaders but not reopen negotiations. Bolger met separately with top union officers, who then went into an evening session with chief federal mediator Wayne L. Horvitz.

Horvitz has been trying, thus far without success, to find a way out of the impasse created by rejection of a tentative contract settlement last week by rank-and-file union members, and by the Postal Service's insistence that the dispute now go to arbitration.

Union officials have not said whether they will obey an anti-strike order issued by U.S. District Court Judge John H. Pratt on Saturday, and wildcat strikes are considered likely, even if the leaders agree to comply.

Bolger's renewed refusal to return to the bargaining table, aired during an interview on "Face the Nation" (OBS, WDVM), drew a sharp denunciation from James L. LaPenta, who was secretary of the postal unions' joint bargaining committee.

"The framework of the law does not in any way stop him from returning to the bargaining table and yet he said that he would not," LaPenta said, charactering Bolger's action as "irresponsible" and indicative of "inexperience in labor relations."

LaPenta also said Bolger was "much too sanguine" in predicting during the interview that postal workers would not strike in the face of a legal ban on walkouts, but denied that he was signaling union members to strike.

LaPenta said there was still hope of averting a walkout so long as talks, even separate meetings with mediators, were under way.

The rejected contract provided for a pay increase of 19.5 percent over three years, considerably less than other major union contracts in recent months, and union officials have said more money would have to be offered to get a postal contract ratified.

In the televised interview, Bolger acknowledged that a strike would have severe and immediate effect, causing "great impact on the financial health of this country" - ranging from corporations that transact business by mail to people who rely on welfare and Social Security checks for their livelihood.

But he said a strike is unnecessary because federal law prescribes fact-finding and arbitration - a course rejected by the NALC and APWU national conventions earlier this summer in favor of a strike if bargaining isn't reopened.

Bolger declined to say how long a strike could be tolerated but suggested a protracted, expensive crippling of Postal Service operations could lead to an end to the government's monopoly of mail delivery.

"I think the people perhaps the Congress would say the Postal Service no longer should enjoy the monopoly they have in delivering letters," he said, noting there already is agitation" for competition from private industry.

He said one recourse in the event of a strike would be to "embargo" handling of mail by the Postal Service, freeing private firms to pick up the business. He said he has the power to do so but would prefer not to.

Contingency plans also provide for a presidential declaration of an emergency, triggering use of federal troops to sort and deliver mail, as they did in an eight-day walkout by about 200,000 postal workers in 1970.

Bolger said specific plans would hinge on she extent and duration of a walkout. He declined to say whether he would fire all strikers, as the law prescribes, but added. "I'm going to continue to uphold the law in every way that's available to me." The Postal Service fired more than 100 workers who struck last month after the contract was negotiated.