Nurses at Prince William Hospital in Manassas voted early yesterday to postpone their strike--originally set for 7 a.m. yesterday--for 12 days to permit a last-gasp effort at negotiating a contract settlement.

But hospital administrator E.L. Derring said later in the day there would be no meeting with the nurses "as long as they have a strike notice."

The nurses, who were unionized in December, said they were under the impression that the hospital would meet with them Aug. 20 if the strike were postponed. The hospital has refused repeatedly to negotiate unless the nurses withdraw their strike notice, which was delivered two weeks ago.

The nurses, members of Local 724 of the Service Employes International Union, were told by their negotiators that at the urging of a federal mediator, the hospital had offered yesterday to bargain if the strike date was postponed until Aug. 25.

The nurses held two meetings Wednesday night and finally voted around 1 a.m. yesterday for the strike postponement.

Either there was a communications mix-up, or there was no offer to meet in the first place.

"If we had known that the hospital had made no offer , the vote would have been much different," said local Vice President Margaret Lloyd.

Lloyd and some other registered nurses interviewed yesterday said they thought the incident was an instance of the hospital trying to double-cross the nurses. "I think it stinks," said Lloyd. "But I must say I'm not surprised. The hospital has been doing this kind of thing all along. I think the community deserves a lot more."

If the nurses do strike, it would apparently be the first such action by registered nurses in Virginia. Sources close to the negotiations say the two sides are still far apart on issues of pay, staffing, and benefits.

The nurses were buoyed late Wednesday afernoon by an announcement that the National Labor Relations Board is planning to issue a complaint against the management of the 154-bed hospital alleging "surface bargaining"--in labor negotiations parlance, just going through the motions of negotiating.

A hearing by the NLRB on the complaint, which is an accusation that the hospital has not bargained in good faith, is expected to be held sometime toward the end of the year. If the hospital is found to have engaged in surface bargaining, and the nurses do go out on strike, then they may be entitled to have their jobs back at the conclusion of the strike.

There are about 130 nurses at Prince William Hospital. All of them are represented by the union -- called the Prince William Healthcare Professionals--but not all of them are actually union members. Thus, it is not known how many nurses would walk off their jobs if a strike were called.

Spokesmen for both sides agree that even if there is a strike, it would not be severe enough to force a shutdown of the hospital.

The hospital's assistant administrator, Kenneth B. Swenson, has said that the hospital will be able to maintain full service in the event of a strike by scheduling extra nurses. But David R. Cromer, an organizer with the SEIU, says he thinks a strike would curtail hospital operations and make conditions "highly dangerous."