Circuit Court Judge James M. Rea intervened yesterday in the increasingly bitter labor dispute in Prince George's County, issuing an order prohibiting 1,500 county workers from striking for the next 90 days.

Shortly after the injunction was issued in Upper Marlboro, however, union officials said they would not rule out the possibility of defying Rea's order by striking as planned next Monday.

"We've defied injunctions before, with grim regularity," said Ernest Crofoot, director of the American Federation of State, County and Municipal Employes, "It looks like the only way you win around here is to take hostages."

County Executive Lawrence Hogan, who personally has directed the county's negotiations and legal maneuvers in Upper Marlboro, said that if the union violates the strike ban "the law will be enforced and any illegal activities will be dealt with appropriately and firmly."

Rea ruled against the union after he decided that an independent labor relations board, which has acted as mediator in the long-sputtering negotiations, did not give the county a chance to properly appeal a recent decision by the board that an impasse existed between the county and the union.

Prince George's County gives public employes the legal right to strike after a complex series of determinations have been made by the labor board, including the finding of an impasse.

Last December the board ruled that such an impasse existed after the two sides failed to reach any agreement following 10 months of bargaining.

In February, county attorneys challenged the impasse finding, arguing that the two sides had met several times since December to discuss contract issues and that changes had been made in their respective positions. They asked the labor board, as a result, to decertify the impasse.

The board refused the county's request after meeting by itself several days later. Rea ruled yesterday that the board should have given the county a chance to argue its request at a public hearing and, since it had not held such a hearing, the county had been denied due process.

After criticizing the board for improper procedures, Rea told a courtroom packed with union members and lawyers for the county: "There will be no strike Monday. I am issuing a 90-day injunction so that the [board] can get itself together. If the board were functioning right, the union today would be in a position to strike."

As Rea announced his decision, which followed a day-long hearing, union representative Paul Manner slumped in his seat, others stared at Rea in disbelief and one of the presidents of the five locals in the union muttered, "Jesus Christ, I can't believe it."

Attorneys for the union said they did not know whether they would appeal Rea's ruling. They said they expected Rea to issue an injunction, but were upset at the length of the ban he imposed.

Manner said that the executive board of the union would be meeting last night to decide on their course of future action. Asked whether the union would consider defying the ban, Crofoot, the union's statewide representative said, "Anything could happen. We've gone against injunctions before and won the strike and paid the fine. This is the only place in Maryland where we've ever been given the legal right to strike."

The court order yesterday brought to a climax a flurry of activity this week by both the union and the county in preparation for the possibility of Monday's strike. Aside from various court hearings on legal issues involved with the strike, Hogan earlier this week announced he had drawn up and signed his own contract with the union -- without ever informing union officials.

The contents of that contract were kept secret until yesterday when Hogan ordered that thousands of letters be printed up for release Friday to county employes detailing elements of his proposed contract and warning workers about the consequences of a strike.

On the basis of the information in Hogan's letters to employes, union officials said the executive's proposed contract appeared to be worse in all ways than a tentative agreement they signed last month with the county that Hogan later vetoed, apparently out of fear that his adversaries on the county council would get too much credit for any settlement.

They also said they thought that many of Hogan's proposals were worse for their membership than the union's old contract, which expired last July.

Since that time, the employes represented by the union, many of whom are the lowest paid county employes, have been working without the cost-of-living adjustments that all nonunionized county employes were given.