Negotiators for the U.S. Postal Service its 660,000 unionized workers failed to settle wage and job security differences by their midnight deadline yesterday, sending the dispute into binding arbitration.

No disruption in mail service was expected; federal law prohibits postal workers from striking.

Marathon bargaining sessions between the Postal Service and its four largest unions proved unsuccessful as disagreements over pay increases and part-time workers blocked a new contract.

"It was a charade. We waited untold hours by the telephone for their phone calls," said Vincent Sombrotto, president of the 234,000-member National Letter Carriers Union, in a midnight news conference.

Postmaster General Anthony M. Frank said in a statement that management had no choice except arbitration because the union's wage demands threatened "to price postal workers right out of their jobs and mail right out of our customers' reach, as all costs are borne by postal customers. Union leadership has failed to recognize the economic realities facing our country and the Postal Service today."

A fact-finding commission will be established, under the direction of the Federal Mediation and Conciliation Service, to review the dispute. It will report back to both sides within 45 days. If management and labor still cannot agree, an arbitration panel will settle the dispute.

Sombrotto said the Postal Service's latest offer was "not only unacceptable, but an insult," because it was an attempt to institute a part-time, lower-paid work force.

The Postal Service wanted the flexibility to hire more part-time workers.

A key stalemate was wages.

The Postal Service had offered lump sum payments of $950 over two years and pay incentives of up to $1,100 a year. The unions demanded 8 percent increases the first year and 7 percent in each of the next two years.

Letter carriers and clerks receive a starting salary of about $24,000 and can earn up to about $31,500. Employees receive an additional $8,560 in benefits and premium pay.