Maryland's highest court dealt a blow today to Prince George's County Executive Lawrence J. Hogan by upholding a ruling that he violated labor laws in a year-long contract dispute with a county public employes union that led to an 11-day strike last summer.

The Court of Appeals ordered Hogan to immediately put into effect a tentative agreement signed nearly a year ago by his own labor negotiators and union officials. Hogan vetoed that agreement after political rivals on the County Council criticized his handling of the protracted labor negotiations with the 1,300-member American Federation of State, County and Municipal Employees.

It was Hogan's refusal to sign the tentative agreement or obey two rulings in the following months that ordered him to implement it that led AFSCME members to walk off the job in August.

The court also ordered Hogan to post notices throughout the county government stating that he violated the law when he vetoed the agreement but that he would not do so again.

Hogan was not available for comment on the decision. His son and top aide Lawrence Hogan Jr. said, "I don't know how he'll react to it." In the past, Hogan's handling of union negotiations has been a sore point for the county executive. He has continually maintained that he acted properly in the labor negotiations with AFSCME despite the two decisions -- by a Public Employees Relations Board and then a county Circuit Court judge -- that he improperly allowed political considerations to enter into the often vitriolic negotiations.

County Attorney Robert Ostrom said he was disappointed by the court decision, which centered on the somewhat technical question of whether the Public Employees Relations Board exceeded tis authority when it ruled that Hogan violated labor laws and ordered him to sign the tentative agreement with AFSCME. Ostrom said the county has 30 days before the Appeals Court decision becomes final and he will take that time to determine how to implement its ruling.

"I am not recommending that we do anything until we give it a careful review," he said. "We have to determine what effect the strike this summer had on the tentative agreement." Ostrom left open the possibility that the county may claim that the strike and an illegal walkout by members of the jail guard union wiped out the terms of the tentative agreement reached last February.

Union attorney Kenneth Niman said he expects Hogan to abide by the terms of the court ruling and hopes that the secretaries, road repair crews and housing inspectors, among others, who are represented by AFSCME, will soon have a contract. The union has been without a contract for two years, although the employes represented by it were granted cost-of-living raises this fall. Niman said he did not know if the court decision will have an effect on the county's decision after the strike to fire some 30 jail guard union members who, he said, walked off the job illegally and whose action on the eve of the strike led to a jail riot.

Union representative Paul Manner said the court ruling would revive his organization, which, because it called of the strike without winning a single concession from the Hogan administration, has been losing members and been deprived of union dues. "The strike hurt; the aftermath of the strike hurt," he said. "We were wounded severely and I think now we can put it together. Larry Hogan won the battle in August but we just won the war."

Today's Court of Appeals decision was the culmination of events that began with quiet negotiations almost exactly two years ago when the county labor negotiators and union officials from five AFSCME locals sat down in an Upper Marlboro office building to come to agreement on a new two-year contract.

After several impasses and the involvement of a federal mediator, Hogan's negotiators and the union on Feb. 12, 1980 signed a two-page handwritten document called a tentative agreement to settle their contract dispute.

Although the county negotiators told the union representatives they "had a deal" with the signing of the document, and felt its terms were quite favorable to the county, Hogan refused to sign it because he was enraged at printed criticism of him by the County Council chairman, according to testimony by one county negotiator. When presented with the tentative agreement, Hogan, surrounded by a bevy of aides, declared that "a strike is sometimes good politics," according to accounts.

Although initially Hogan publicly said he had not signed the agreement bacause of some minor health and economic benefits, within a few days he said the agreement was unacceptable because it provided too many county-paid benefits to union leaders. Pay raises for union members had been agreed upon by both sides and were not an issue in the dispute.

The union challenged Hogan's explanations and filed an unfair labor practice charge against him, a charge that was upheld by the Public Employees Relations Board and, when appealed by the county, by Circuit Court Judge Jacob Levin. When Hogan refused to implement either order but instead appealed them to the state courts, the union went on strike.