A board of outside mediators yesterday ordered the postponement of a strike by 1,500 Prince George's County public employes until March 24 at the earliest.

The union representing the employes, most of whom are among the lowest paid in county government, previously had notified the county that its members would walk off the job Tuesday unless its year-old contract dispute with the county was resolved.

Prince George's County is the only jurisdiction in the Washington area with a labor code that grants public employes the right to strike.

Before those employes can strike, however, they are required to file a 10-day notice of intent with the county government. The union filed such a notice last week but the county, in one of a series of legal maneuvers designed to forestall the impending job action, contested the notification, saying it did not include all the information required by law.

The mediators ruled in favor of the county yesterday and, after the union provided the required additional information, ordered the 10-day strike countdown to begin again today.

If the union members walk off the job Monday, March 24, it will be the first such strike in Prince George's County history. Landfill operators, jail guards, secretaries and building inspectors, among other employes, would possibly leave their jobs in the event of a strike.

The union, which is part of the national American Federation of State, County and Municipal Employees, has been negotiating a contract with the county for the last year.

Several weeks ago, the negotiations erupted into public view when County Executive Lawrence Hogan vetoed a tentative agreement with the union, apparently because he felt it would look as though he had been pressured into it by his political adversaries.Hogan has since said that some economic issues in the agreement were unacceptable to him.

After Hogan vetoes the agreement, which would have given the public employes a 4.7 wage increase, the union accused Hogan of bargaining in bad faith and filed an unfair labor practice charge against the county.

The union's unfair labor practices charge has not yet been ruled on by an independent hearing examiner.

This week several other county unions came out in support of the public employes, promising financial assistance and, in some cases, work-to-the-rule job actions if the county does not settle with the union.

The president of the 800-member Fraternal Order of Police said that his union would not allow its members to be used as "strikebreakers," and would refuse to cross picket lines if Hogan "persisted in bad faith bargaining."

The presidents of the Prince George's Educators Association, Fire-fighters union and school support staff union, who represent a total of about 132,000 county employes, all pledged yesterday to support the public employes strike if it occurs. And the president of the 230-member Police Civilian Employes association, which has reached an impasse in its own year-long contract dispute with Hogan, said yesterday his union may notify the county today that it intends to strike in 10 days.