Postmaster General William F. Bolger said yesterday he will ask the National Labor Relations Board to determine which of the four unions representing nearly 600,000 postal employes should negotiate a new labor contract with the U.S. Postal Service.

Bolger's surprise announcement came one week before contract talks were to begin.

His action means the talks could be delayed indefinitely.

Bolger said his action is necessary because he is faced with the prospect of working out contracts in an "unworkable" collective-bargaining structure involving competing unions.

However, Morris (Mole) Biller, president of the American Postal workers Union, said Bolger's announcement was "the worst kind of deceit."

"We've been working with them, moving towards these negotiations for more than six months," Biller said of preliminary talks with postal officials. m"Never in any of those talks did any of them indicate they would do something like this."

Vincent Sombrotto, president of the National Association of Letter Carriers, called Bolger's announcement "asinine . . . designed to undermine the collective-bargaining process."

Sombrotto's union and the APWU represent about 500,000 of the organized postal workers. The other 100,000 are represented by the Mail Handlers' Division of the Laborers' International Union and the National Rural Letter Carriers' Association.

From 1971 to 1975, all four unions bargained as a single unit with the Postal Service and signed a common contract. In 1978 talks, the rural letter carriers' union split from the other unions and bargained separately. This year, with the factionalism among the four groups growing, the two largest unions -- the Apwu and the NALC -- planned to coordinate bargaining, while the two smaller groups, planned to negotiate individually.

Bolger, who plans to file his NLRB petition Friday, said, "Jurisdictional disputes between these unions have led to harsh words, frayed feelings and active raiding campaigns . . . . Such animosity . . . reduces the possibility of reaching sound agreements."