Union nurses on strike against Prince George's County-owned hospitals voted last night to remain off the job until their demand that all nurses be allowed to return to their old jobs is met.

Although the nurses had been expected to vote on the tentative agreement worked out during a marathon bargaining session that lasted until early yesterday, union President Carolyn Larkin said no formal ratification vote was taken.

Instead, the nurses voted unanimously to have negotiators continue bargaining until their conditions for returning to work have been met, Larkin said.

At issue are 22 specialty nursing positions that management has filled with nonstriking and newly hired employes since the nurses went on strike Aug. 29. Management said striking nurses, who previously held those jobs, would be assigned other jobs.

Mike Canning, spokesman for the corporation that runs the three hospitals, said last night, "We've guaranteed them all a job at the same rate of pay, but we can't return those 22 to their old jobs. Legally, we can't do that."

The strike by 500 nurses of the Prince George's Staff Chapter of the Maryland Nurses Association has affected Prince George's General Hospital, Greater Laurel-Beltsville Hospital and the Bowie Health Center. The hospitals are operated by Community Hospital and Health Care Systems Inc.

Both Larkin and Canning said that most of the contract issues were resolved during talks that resumed Friday, before negotiations broke off about 3 a.m. yesterday. Talks are expected to resume Monday morning.

Larkin said that the 300 nurses who attended last night's meeting "were given a strong overview of the fact that . . . if we held out for the last 22 nurses that it could be a long, strenuous struggle," Larkin said.

"The support was just overwhelming, even after having been read the worst possible scenario," Larkin said.

Larkin accused management of blocking agreement and seeking its "pound of flesh" by retaining 15 nonstriking and seven newly hired nurses in the specialty positions, including those in emergency and intensive care units.

Instead of filling the 70 vacancies that existed before the strike, she said, management "is holding firm to punish all of the nurses for having to go out on strike."

Canning said the latest proposal by the corporation includes a 35-cent-an-hour raise retroactive to Aug. 1, to be followed by another 35-cent-an-hour raise in February 1984. This represents about a 7 percent increase. The nurses, who now are paid between $17,000 and $25,000 annually, had asked for an 11.5 percent increase.

According to Canning, the newest proposal also led to the creation of a committee to review the role of professional nurses and allows paid time off for nurses who are union representatives to conduct union business.

The private, nonprofit corporation that took over management of the health facilities on July 1 asserted that the nurses' salaries compared well with nurses at other Maryland hospitals.