Publisher Rupert Murdoch has borrowed a union bargaining tactic and issued an ultimatum to his 450 news and commercial employes at the New York Post.

Labor unions traditionally have set strike deadlines to speed lagging negotiations.

Murdoch set midnight April 14 as a deadline for resolution of all dispute with the Newspaper Guild, which represents those workers. He did not specify whether the employes would be locked out if the deadline is not met.

A spokesman for Murdoch, Martin Fischbein, said the ultimatum was necessary because "absent a deadline you can't get people to concentrate their minds."

"We are trying to make changes," Fischbein said. A major change which the Post management says is essential to the survival of New York's only afternoon paper iis a 25 to 35 percent reduction of the work force in the news and commercial departments.

"You cannot do a layoff retroactively," Fishbein said in explaining why the Post took the unusual step.

"The viability of the paper is at stake. We've been quite clear about that Fishbein said.

Harry Fisdell, executive vice president of the Guild's New York local, and Barbara Yuncker, chairman of the Guild's Post unit, said the union has not set a strike deadline although its control expired March 31.

They said that if the Post breaks off negotiations and puts out a non-union produced newspaper, its management will be responsible for any consequences, including the demise of the paper.

Fishbein said that four newspapers, which had about 4,500 jobs and one million readers, have folded in New York since 1963. "our friends in the labor movement must share responsibility for that," he said.

"This is what is at stake," Fischbein said. "Mr. Murdoch doesn't intend to make the Post the fifth paper."

Murdoch took over the Post a little more than a year ago. The paper is reported to be losing around $150,000 a week. Attempts to increase advertising have not succeeded so management is seeking to cut costs.

The Post wants to dismiss 25 to 35 per cent of its Guild employes without following the "last-hired, first-fired" practice which the union insists upon. The Post wants to fire those it finds "incompatible with the new management's publishing concept."

Union leaders call that demand the "Auschwitz clause."

During each of the last two weekends, the Post produced dummy papers without any union labor.

"We are now confident of our ability to put out a paper with no unions in the shop," Fishbein said. He added, however, that in any labor confrontation management expects some unions to cross other unions' lines and work.

Yuncker said that the Guild has been pledged the support of seven unions whose contracts expired when the Guild's did. Only the union representing the drivers has not promised its support, she said.

Most important in the Post's plans to publish without some union workers is its confidence that the Newspaper and Mail Deliverers' Union will delivre the Post even if there is a strike or lockout involving other unions.