The nation's two largest postal workers unions broke off contract talks with the Postal Service yesterday, charging that a settlement announced Tuesday with a third, smaller union included a secret deal that transfers jurisdiction of 10,000 jobs from one of the larger unions to the small one.

The Postal Service and the third union, the Mail Handlers Division of the Laborers' International Union, refused to comment. Talks with the two larger unions broke off four days before the deadline for an accord.

Vincent R. Sombrotto, president of the National Association of Letter Carriers, and Moe Biller, president of the American Postal Workers Union, said they will return to the bargaining table only if Postal Service negotiators guarantee that the third union's contract does not alter union jurisdiction.

"I have no specific comment about the issue itself," the Postal Service's chief negotiator, Thomas J. Fritsch, said in a statement. "However, management stands ready and willing to bargain at any time."

A federal mediator may intervene today, but if there is no agreement before midnight Monday, union leaders said, the contract probably will be resolved by binding arbitration. Federal employes are prohibited from striking.

Biller and Sombrotto said an informant had told them earlier in the day that the Mail Handlers accord would reclassify some clerks as mail handlers. The reclassification would shift some Postal Service jobs from Biller's American Postal Workers Union to the Mail Handlers unit of the Laborers' union, they said.

Sombrotto said his union joined in breaking off negotiations to show solidarity with the Postal Workers. The Postal Workers and the Letter Carriers have negotiated together, separately from the Mail Handlers, since contract talks began in April.

Postal Service and Mail Handlers officials would not disclose the terms of their three-year settlement. Biller and Sombrotto said the Mail Handlers, who represent about 51,000 workers, agreed to an average salary increase of 1.6 percent a year and made major concessions on overtime pay.

The Letter Carriers and the Postal Workers unions, who represent 575,000 workers, on Tuesday called for a 6.8 percent salary increase and upgrades in other benefits.

"To get the Mail Handlers to agree to that sweetheart arrangement, there had to be some sweet offer," Sombrotto said. "That sweetness was 10,000 jobs."